I have created a manual trigger rule to create a linked issue on another project, taking some of the fields of the original issue.
I try to populate summary, priority, and 3 customer fields(Operator, Vendor, Regulation). It populates only 1 of the custom fields (Regulation). The populated one is Multiple choice, while those that fail are Label type.
Both projects have all 3 custom fields, both projects have permissions for the automation users.
The error message is telling you that the fields do not exist on the issue you are trying to update.
Either you're giving the automation the wrong fields to work with, or the fields do not exist for that project and issue type. Try editing the issue to see if the fields are there.