Need help with the error related to decode and returncode.

When I run the below query in 12c, it runs fine, but same gives error in 19c.

We recently migrated from 12c to 19c, so modified some tables to point to unified_audit_trail. Need to modify some scripts to enable auditing. Need help in modifying this to correct one so it can run in 19c DB.

select

 to_char(a.event_timestamp,'MM/DD/YY HH24:MI:SS')||', Action: '||

 action_name||' on '||DBUSERNAME||'.'||object_name||' by '||DBUSERNAME||' (OS_USER='||os_username||')'||' to '||grantee||

 decode(returncode,0,null,' - Failed') info