In the cases users are unable to add API to a specific application in API Developer Portal.
All Supported versions of API Developer Portal.
Duplicate UUID for the Application in APPLICATION_API_API_GROUP_XREF table.
Run this following query to find any Duplicate UUID for the Application to which the API is being assigned.
mysql> SELECT * FROM APPLICATION_API_API_GROUP_XREF WHERE APPLICATION_UUID = '';
---------------
Run this query against portal database, with the duplicate UUID from the above query.
mysql> DELETE FROM APPLICATION_API_API_GROUP_XREF WHERE UUID = '';
Now, try to add the API to the Appilcation.