GRANT User Permission to Database

[Derrick Kearney]

There is obviously something odd with GRANT in version 8, I used this command with MySQL 5.6 and it worked perfectly. It is simply not working as expected in MySQL 8.0. There are 2 questions on StackOverflow with a wall of guesses. Why would, GRANT ALL ON simple_db.* TO 'app_user'@'localhost'; return "ERROR 1410 (42000): You are not allowed to create a user with GRANT"
I am not trying to create a user, the user already exists. I read the documentation but it is just not working, Can someone please tell me how to correctly grant privileges to user X for database Y?