Updating my phone

To add to KiltedTim’s response:

larenze wrote:

I even bought 50 GB to add storage to my phone,

You cannot add storage to your iPhone. You bought iCloud storage, which is completely different.

About storage on your device and in iCloud - Apple Support

You also need to tell us your iTunes version number.

There is so much you have not told us, it would be impossible to guess why.

There are at least many tens of possible reasons.

Update your iPhone, iPad, or iPod touch - Apple Support

You still haven't told us what happens when you try...

Try updating using iTunes instead of over the air.