https://repost.aws/en/knowledge-center/redshift-user-cannot-be-dropped -- v_generate_user_grant_revoke_ddl.sql

That means you (or someone else) already created that view. Try "select * from admin.v_generate_user_grant_revoke_ddl" it'll probably return a result. They recently added some permissions changes (https://docs.aws.amazon.com/redshift/latest/mgmt/cluster-versions.html#cluster-version-183) and probably modded this article/view to incorporate those so that you can clean up scoped permissions. Just drop the view, then run the latest version of the that "create or replace view" sql and it should work fine.