It's ridiculous

I'm a little frustrated. In every, even the simplest one project, I meet issues.

1. How to make the card right in the middle? Using Display Flex and Justify-Content on body works OK, but Align Items doesn't. I don't think that I must use margin-top.
2. Why my image doesn't fit to main? Logically when I set its height to 100% it should cover the white box (main) but it doesn't. Everytime I have no problems with mobile design, but as soon as I swap to desktop it changes to a nightmare. I know what to do in theory but it just doesn't work.

Thanks for help.