Expression evaluation error at function a!imageField [line 53]: The image at index 1 in an image gallery component [label=""] has an invalid value for "document". "document" must not be null.

intro: i did not do the specific exercise but some general information

the error tells you already the issue :) ri!vehicle, your recordtype is a null value
In result you don't have a value for vehicle.image neither there

a!documentimage wont work properly with a null or empty value for the "document:" paramater
you need a  picture aka document in your application and the ID of the document needs to be the value of ri!record[AC_vehicle.image] 


perhaps this link helps you:
docs.appian.com/.../Document_Image_Component.html

edit: you can try the following in the test parameters of your rule input:
docs.appian.com/.../reference-records.html

Ok this all makes sense! What I don't understand is that it hasn't asked me to add images so how do I get them ? Sorry as I am a beginner!

step 1:
you create elements of the dype "document"

There you can select or drag and drop mutliple images

After that you need to select the name and folder and if a constant shall be created automatically.

This is how an uploaded image looks like. You see the name, a preview, type, size, folder, description etc.
AND the ID. This is the ID you need for the document parameter at a!documentimage() and which needs to be the value of [vehicle.image]

step 1:
you create elements of the dype "document"

There you can select or drag and drop mutliple images

After that you need to select the name and folder and if a constant shall be created automatically.

This is how an uploaded image looks like. You see the name, a preview, type, size, folder, description etc.
AND the ID. This is the ID you need for the document parameter at a!documentimage() and which needs to be the value of [vehicle.image]