In 12.5.4 I get the same message as in 15.7.
Just to be sure: you want that any user can do an Insert on dbo.amd_abo_clt, except those belonging to group (role) IT_READONLY ?
I've never seen this setup, so I doubt very much that this is possible.
Revoke works for previously granted statements.
In your example, you can only revoke the permission from public.
Luc.