This error can occur if your approval process is set to select approver from view. In this case, we would expect the '{OBJECT_NAME}' to be like 'UDEF_PR_Approver' with a number at the end or an underscore followed by a number.
You can check and amend this using the following steps:
Make a note of the PR Type that gives the error and note the '{OBJECT_NAME}' from the error message.
As an admin user, login to FinOps Admin.
Click System Options then click Purchasing.
Click Purchase Request Setup.
Select the PR Type then click Edit.
Select the Approval tab then select Stage 1.
Click Edit.
If the Approver Type is Selected From View go to step 9 if not, exit stage.
If the View is your '{OBJECT_NAME}', click the ellipses button and scroll down the list to find the correct view.
Repeat steps 7 to 9 on all stages until your '{OBJECT_NAME}' is found and corrected.
If you still get the error message please raise a new case. Please include full details of the error message received including the '{OBJECT_NAME}' and mention this article.