installing linuxmint cinnamon 64 bit
 

I had version 19.1 and installed version 20.2.
By starting system comes txt: GNU GRUB. If I push by starting F12 and by chosing P1:HL-DT-St-DVDRAM GH24NSCO and "enter" I'm in. What is going on wrong by installation?

most probably your grub configuration is incorrect. I guess there is an error message somewhere. But anyway if you are in you can [re]configure your grub.
see for example: https://itsfoss.com/update-grub/

While in Mint 20.2 go to Download manager and get Boot Repair and run it. That should fix the issue. Good luck

I don'nt have Download Repair in my Mint 20.2, sorry

To make this as simple as I can.... Go to main menu....hover over the large icons software manager should be the second one down. Click it......when it pops up look at the bottom and select System tools. then scroll down until you find Boot Repair. hit install, it will ask for your password....type it in and it will download and install. Get used to this Software manager you will use it often. it will then be in your Administration folder when you click it just go with what it asks. Bingo the Software Manager is a noobies best friend. And remember no one was born an expert.