Appian Step by Step - Process modelling 101 - Step 10 (part 2, page 4)

Hi Martin Rodriguez   couple of questions from my side

• Where are you doing this in the interface Rule inputs?
  If yes, then you are mapping it in the wrong way. The field you use to save the value must be configured in its value and saveInto fields with recordType!yourrecord.relationship.relatedrecord.fields.fieldValue.
• Did you establish mapping between the parent record and the child record?

Hi! I'm doing it when I drag and drop the previous interface I previously created (W#SA_VehicleDetailsView) into the Vehicle Details section. That's when the Configure Inputs dialog appears and I'm supposed to write the expression that you see in the screencapture.
Also, when typing 'relationships' the only options I'm given are 'eventHistory' and 'subscriber'

Need see your interface code please 

W0112SA_MaintenanceRequest 

From the above image, you can see there is a tab called expression, could you click that tab on your interface, and post the code please 

a!formLayout(
label: "New Maintenance Request",
contents: {
a!sectionLayout(
contents: {}
),
a!sectionLayout(
label: "Vehicle Details",
contents: {}
)
},
buttons: a!buttonLayout(
primaryButtons: {
a!buttonWidget(
label: "Submit",
submit: true,
style: "SOLID"
)
},
secondaryButtons: {
a!buttonWidget(
label: "Cancel",
value: true,
saveInto: ri!cancel,
submit: true,
style: "OUTLINE",
validate: false
)
}
)
)

Can you open  check the record type? W0112SA_MaintenanceRequest  is that is having any relationship

Yes, however the record type is W0112SA Maintenance, W0112SA_MaintenanceRequest is the name of the interface.

Yes, however the record type is W0112SA Maintenance, W0112SA_MaintenanceRequest is the name of the interface.