I upgraded I cloud to 50gb but I can’t install new version 17.2.1

Buying more iCloud storage does not increase the phones storage space, it is a fixed size.

You can either try deleting stuff on your phone to free up space or try using a computer to perform the upgrade. Using a computer to upgrade requires less storage space on your phone.

Here are some helpful articles that can guide you on how to free up storage space on your phone.

How to check the storage on your iPhone and iPad - Apple Support

Manage your photo storage - Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

Update using your computer -

Click on this link for instructions on how to update your phone using a computer

Update iOS on iPhone - Apple Support