Have you followed the steps listed carefully? https://developer.themoviedb.org/reference/authentication-how-do-i-generate-a-session-id

I found there is an error in documentation. If you want to create a session with login, first request the token, after that validate that token (In documentation appears as create session with login, this is the error). After that you can create a session as described in step 3 with the validated token and you'll receive the session id.

Hmhm, im having the same issue while trying to add a movie into a list, it works fine running locally, when i deploy my service i keep getting this error:

status: 401 { success: false, status_code: 3, status_message: 'Authentication failed: You do not have permissions to access the service.' }