cannot fill input to the form

Can you post the part of the code where the error comes from?

Better to use the index() function for indexing objects as suggested in the error message. 

validations: if(todate(ri!vehicle.vehicleNextMaintenanceDate) <
todate(ri!vehicle.vehicleLastMaintenanceDate),
"The next maintenance date must be after the last maintenance date.",
null)

),

Please put the code of the component you are using, not only the validation part.

Also, what type of object is the ri!vehicle? Is it a CDT or an array ?

If you are working on the Acme application example I guess the ri!vehicle is the CDT:

'type!{urn:com:appian:types}AA_Vehicle'()

In this CDT the vehicleNextMaintenanceDate and vehicleLastMaintenanceDate fields are of type Date so there is no reason to cast them again into date type:

todate(ri!vehicle.vehicleNextMaintenanceDate) < todate(ri!vehicle.vehicleLastMaintenanceDate)

a!localVariables(
local!category: rule!AA_GetReferenceData(type: "Category"),
local!status: rule!AA_GetReferenceData(type: "Status"),
local!condition: rule!AA_GetReferenceData(type:"Condition"),
a!formLayout(
label: "ADD VEHICLE",
contents: {
a!columnsLayout(
columns: {
a!columnLayout(
contents: {
a!textField(
label: "Model",
labelPosition: "ABOVE",
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{54792ae7-b982-4e9b-a065-38f833520e00}vehiclemodel'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{54792ae7-b982-4e9b-a065-38f833520e00}vehiclemodel'],
characterLimit: 55,
required: false
),
a!textField(
label: "Make",
labelPosition: "ABOVE",
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{ee759ec4-7f80-43f0-9e33-ff60699dadc8}vehiclemake'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{ee759ec4-7f80-43f0-9e33-ff60699dadc8}vehiclemake'],
characterLimit: 55,
required: false
),
a!textField(
label: "Color",
labelPosition: "ABOVE",
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{f931fba7-4fbf-430e-8ed0-f733dafe5b17}vehiclecolor'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{f931fba7-4fbf-430e-8ed0-f733dafe5b17}vehiclecolor'],
characterLimit: 55,
required: false
),
a!integerField(
label: "Mileage",
labelPosition: "ABOVE",
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{41847dfb-6e57-46d9-9a11-1c793dd9ee44}vehiclemileage'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{41847dfb-6e57-46d9-9a11-1c793dd9ee44}vehiclemileage'],
required: false
),
a!textField(
label: "VIN",
labelPosition: "ABOVE",
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{a8f2de8e-f628-4515-ab55-360341cdbbca}vehiclevin'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{a8f2de8e-f628-4515-ab55-360341cdbbca}vehiclevin'],
characterLimit: 17,
required: false
),
a!dateField(
label: "Next Maintenance",
labelPosition: "ABOVE",
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{f3c174c0-2185-4448-95c2-0b66847d3de5}vehiclenextmaintenancedate'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{f3c174c0-2185-4448-95c2-0b66847d3de5}vehiclenextmaintenancedate'],
required: false
)
}
),
a!columnLayout(
contents: {
a!integerField(
label: "Year",
labelPosition: "ABOVE",
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{9f20cb6e-bcf5-4fc8-9f4d-0ae08b633c67}vehicleyear'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{9f20cb6e-bcf5-4fc8-9f4d-0ae08b633c67}vehicleyear'],
required: false
),
a!dateField(
label: "Last Maintenance",
labelPosition: "ABOVE",
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{3eaef3a8-ad19-4e10-bbf4-b07b6944ab3e}vehiclelastmaintenancedate'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{3eaef3a8-ad19-4e10-bbf4-b07b6944ab3e}vehiclelastmaintenancedate'],
required: false,
validations: if(todate(ri!vehicle.vehicleNextMaintenanceDate) < todate(ri!vehicle.vehicleLastMaintenanceDate),
"The next maintenance date must be after the last maintenance date.",
null)

),
a!dateField(
label: "Date Added",
labelPosition: "ABOVE",
value: today(),
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{b720b92c-7bba-4ff2-bb4d-e44852afee63}vehicledateadded'],
required: false,
readOnly: true
),
a!dropdownField(
label: "Category",
labelPosition: "ABOVE",
placeholder: "--- Select a Value ---",
choiceLabels: local!category['recordType!{0063da9e-52e1-4f6e-af28-f0fa2eefe66f}Category.fields.{142b6584-3be7-42c3-852f-600471549007}label']
,
choiceValues: local!category['recordType!{0063da9e-52e1-4f6e-af28-f0fa2eefe66f}Category.fields.{94d01ec3-9cb9-412c-833a-268fe696c610}id'],
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{7ad2c969-c93d-4915-b1e4-1a69b95de838}vehiclecategory'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{7ad2c969-c93d-4915-b1e4-1a69b95de838}vehiclecategory'],
searchDisplay: "AUTO",
required: true,
validations: {}
),
a!dropdownField(
label: "Status",
labelPosition: "ABOVE",
placeholder: "--- Select a Value ---",
choiceLabels: local!status['recordType!{359c9a34-94c4-4e7f-aa46-54f9b6b5a454}Status.fields.{c192b0cc-7bdf-4bf9-aea4-eca350786546}label'],
choiceValues: local!status['recordType!{359c9a34-94c4-4e7f-aa46-54f9b6b5a454}Status.fields.{8861f86e-a4ab-4e54-9c51-75fc2cad210d}id'],
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{9be749ac-1fbf-42bf-bc6b-7ab58c11bf3c}vehiclestatus'],
saveInto: {},
searchDisplay: "AUTO",
required: true,
validations: {}
),
a!dropdownField(
label: "Condition",
labelPosition: "ABOVE",
placeholder: "--- Select a Value ---",
choiceLabels: local!condition['recordType!{b86e3be8-f076-4bf4-9f37-ba3d2ecd6815}Condition.fields.{1b84c63a-9805-4988-844c-2bb03b83abe8}label'],
choiceValues: local!condition['recordType!{b86e3be8-f076-4bf4-9f37-ba3d2ecd6815}Condition.fields.{44e3243e-afcf-4fe8-a200-853d2d522703}id'],
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{0c4fee03-ae4c-4d1e-8aff-a81b01dc833d}vehiclecondition'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{0c4fee03-ae4c-4d1e-8aff-a81b01dc833d}vehiclecondition'],
searchDisplay: "AUTO",
required: true,
validations: {}
),
a!fileUploadField(
label: "Image",
labelPosition: "ABOVE",
target: cons!PA_DOCUMENTS_FOLDER_POINTER,
maxSelections: 1,
value: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{b9c5efac-ec91-4a62-ba1d-0528930b4f9c}vehicleimage'],
saveInto: ri!vehicle['recordType!{daaebabb-0af7-4cd6-b5ae-12bff43fe006}PA Vehicle.fields.{b9c5efac-ec91-4a62-ba1d-0528930b4f9c}vehicleimage'],
required: true,
validations: {}
)
}
)
}
)
},
buttons: a!buttonLayout(
primaryButtons: {
a!buttonWidget(
label: "Submit",
saveInto: a!save(ri!vehicle.vehicleDateAdded, today()),
submit: true,
style: "PRIMARY",
validate: true
)
},
secondaryButtons: {
a!buttonWidget(
label: "Cancel",
value: true,
saveInto: ri!cancel,
submit: true,
style: "NORMAL",
validate: false
)
}
)
)
)

fyi: please put super-long snippets of code (or, any snippets of code) in a Code Box - this both saves the length of the thread from getting blown out of the water by single comments that take up screenfuls of room, as well as preserving indentation (as well as using a friendlier, monospace font for reading).

fyi: please put super-long snippets of code (or, any snippets of code) in a Code Box - this both saves the length of the thread from getting blown out of the water by single comments that take up screenfuls of room, as well as preserving indentation (as well as using a friendlier, monospace font for reading).