I am using next-auth in supabase , I have user table and Idea table as one-many relationship , I tried to create policy for delete own user idea but I couldn't here is my condition for deleting next_auth.uid() = userId

Assuming your function works you have to put names (columns/tables) in double quotes if they have capital letters to use them. You should NOT use capitals in Postgres for this reason, although it is permitted.