Please check the version and raise an SR along with details.
Invoice Validation (APPRVL) Program Errors ORA-00001: unique constraint (AP.AP_SELF_ASSESSED_TAX_DIST_U1) violated (Doc ID 1538735.1)
SR responded the new invoice functionality will allow user to enter invoices with same supplier and same invoice number and different sites.
The index is coming from old 11i release, so after migrating to 12.2.x, the index need to be dropped.
Apparently, it is possible 11i object remains in R12 unsupported after upgrade and users will decide if the object should be dropped manually or not.
In this case, the unique constraint message caused the confusion to our users. We decided to drop the index per Oracle support.
Nice to hear you are able to resolve this issue.
Please marked this as answered.