Postgres 15. permission denied for schema public

The first comment nailed the most likely reason this is happening. Quoting the release announcement:

PostgreSQL 15 also revokes the CREATE permission from all users except a database owner from the public (or default) schema.

The reason your fix didn’t work is that all actions you took on database postgres in regards to user admin‘s privileges on schema public concern only that schema within the database postgres. Schema public on database postgres is not the same schema public as the one on newly created mydb.

Also, this:

GRANT ALL ON DATABASE mydb TO admin;

grants privileges on the database itself, not things within the database. admin can now drop the database, for example, still without being able to create tables in schema public. My guess is that you wanted to make admin also the owner of mydb, in which case you need to add

ALTER DATABASE mydb OWNER TO admin;

Or you need to repeat your GRANT USAGE, CREATE ON SCHEMA public TO admin; on mydb.

Here’s some more documentation on secure schema usage patterns the PostgreSQL 15 change was based on.