Logged into the database base as sys, then created a user A and tried to grant him insert privileges to table test which exists in schema B.

SQL> grant insert on B.TEST to user A;

I get the error insufficient privelages.

So I logged into the schema b and gave the privileges to the user A, which succeded. Not sure why I could not grant the insert privileges against another schema to a user, if I am logged in as sys or system.