I created a new database.
I created new user with CREATE USER UNAME PASSWORD 'PASS' GRANT ADMIN ROLE;
I logged in with newly created account, specifying RDB$ADMIN role.
I tried to create a new user with CREATE USER UNAME1 PASSWORD 'PASS1' GRANT ADMIN ROLE;

The error is:

Unsuccessful execution caused by system error that does not preclude successful execution of subsequent statements.
add record error.
no permission for insert/write access to TABLE USERS.

Apparently, my newly created user UNAME should have SYSDBA privileges, but actually does not have them.