Postgresql throws an error on execute query, what's the problem?

Question

Python

Good afternoon, I want to display a list of all users from a Postgresql table
Output code:

sql.execute('SELECT count(user_id) FROM main')
users = sql.fetchall()

The problem is that Postgresql gives an error
psycopg2.ProgrammingError: no results to fetch
Sometimes it happens that postgresql gives the number of users correctly.
What is the reason for this? And how to solve it?