ORA-01031

ORA-01031 Insufficient privileges

I am trying to generate FK on table in schema A, which is the reference of PK on another table in schema B. The owner of schema A is also a DBA. It gives an error ORA-01031 Insufficient privillages

oar-01033

ORA-01033 and from I understand from reading posts..i'm to log in as sys/password as sysdba. When I do this, sqlplus is returning the error message: ORA -01031 Insufficient privileges. I've tried several different usernames...but SYS should work as i've used this many many times in the past "as...