We faced a simliar error before
this was because we created a table called SYS,
so look for any object called sys *you can do so by select from DBA_OBJECTS where name ='SYS'
if any object is found rename it.

or may be you ran catalog scripts as user system not user SYS, for example, while you were running the scripts that creates the replication repository, you logged as system, or as another user.

I hope this will solve your problem.