When starting up a standby database in open mode I noticed the following error in the alertlog:
Unable to obtain current patch information as the database or pluggable database was opened for read-only access.
Dumping current patch information
Unable to obtain current patch information due to error: 16000
SQL> select dbms_sqlpatch.verify_queryable_inventory from dual;
Unfortunately there is no fix for the time being as of now, but the following bug is already being worked by the development team:
Bug 21098077 - PACKAGES USED TO OBTAIN DATABASE PATCH INFORMATION SHOULD HANDLE READ-ONLY DB
Meanwhile, This error is minor and doesn't indicate any problem with the database, so it can be safely ignored.