I started as Brandenburg in year 1444 (standard date) and were to look at the requirements for creating Prussia and its prussian monarchy, but the option just states that Brandenburg will turn into Prussia under an absolute monarchy instead of the prussian monarchy.
Then I decided to look at a new game with a new starting date when prussia was already formed, and it had the prussian monarchy government. So something is wrong about the option to turn into Prussia but not when you already are Prussia.