Try to connect to RMAN catalog with the following errors:

PL/SQL package SYS.DBMS_BACKUP_RESTORE version 19.20.00.00 in TARGET database is not current
PL/SQL package SYS.DBMS_RCVMAN version 19.20.00.00 in TARGET database is not current
connected to target database: TESTDB (DBID=1231118758)
connected to recovery catalog database

CAUSE

Once the DBRU/DBRUR is applied  datapatch was not run. While DBRU/DBRUR will update the binary datpatch would ensure the dictionary are also updated

SOLUTION

Crosscheck if datapatch was run.

SET PAGESIZE 100
SET LINESIZE 300
COLUMN status FORMAT A10
COLUMN description FORMAT A40
COLUMN source_version FORMAT A10
COLUMN target_version FORMAT A10

select CON_ID,
TO_CHAR(action_time, 'DD-MON-YYYY HH24:MI:SS') AS action_time,
PATCH_ID,
PATCH_TYPE,
ACTION,
DESCRIPTION,
SOURCE_VERSION,
TARGET_VERSION
from CDB_REGISTRY_SQLPATCH
order by CON_ID, action_time, patch_id;

Run datapatch if it was not run:

$ORACLE_HOME/OPatch/datapatch -verbose

select * from pdb_plug_in_violations where status <> 'RESOLVED';

$ORACLE_HOME/perl/bin/perl $ORACLE_HOME/rdbms/admin/catcon.pl -n 1 -e -b utlrp -d $ORACLE_HOME/rdbms/admin utlrp.sq