Value in the field does not exist does not exist in the datasource app

I’m receiving the below error.

I have a dropdown in my contact form that fills in a lookup field in the table, getting the lookup data from another table.

This worked until I made some changes a few months ago, and I cannot remember what I need to do wo make this work. Do I need to add the App Ids and API tokens of both apps in the App Id and API Token fields or something?

A value VALUE in the field FIELDNAME does not exist in the datasource app for lookup, or you do not have permission to view the app or the field.
array (
0 => ‘code: 520’,
1 => ‘message: ‘,
)