NULL is a single row comparison value - so you can't use it to compare against a result set - only single values. Try using EXISTS:

SELECT P.* FROM TABLE1 P
WHERE NOT EXISTS (SELECT CP.COLUMN1 FROM TABLE2 CP WHERE CP.CAT='PSTATUS')
        OR P.ID IN (SELECT CP.COLUMN1 FROM TABLE2 CP WHERE CP.CAT='PSTATUS')

See here for more details on Exists:
https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/EXISTS-Condition.html

and NULL checking : https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/Null-Conditions.html

If this doesn't help, please provide more details of the data, with your desired result. HTH< NIck