You haven't actually clarified whether your screenshots are the right dimensions.
You've said:
I already tried to cut it into the right form for example 6,5" for the biggest screen.
What are the dimensions of the screenshots that App Store Connect is asking you for? A 6.9" iPhone 17 Pro Max, for example, should be 1320 x 2868. When you add those they will be resized by App Store Connect to fit all other iPhone sizes.
What are the dimensions of the screenshots you're trying to upload?
App Store Connect will tell you exactly what's wrong with your screenshots, so please copy that error message and paste it here (as text, not as an image, please).
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect