I had this same issue and beat my head against a wall for the longest time. The answer is indeed in the Package_Repositories page, but its not as explicit as you'd like. I had to edit my repositories list at /etc/apt/sources.list as stated in the post, but you also have to comment out the line listed in /etc/apt/sources.list.d/pve-enterprise.list. If you don't comment out the line in the second file, it still tries to use the enterprise repo. After commenting that out, run apt update and it should work.