I bought storage but I cannot update my iPhone

You may be confused about storage.

Storage on an iPhone cannot be increased. It's fixed. The only way to create more space on an iPhone is start deleting apps, music, videos, texts, documents, etc.

You may have purchased more storage on iCloud, so you can store backups and other data on iCloud.

But, buying more storage on iCloud does nothing for the storage capacity of your iPhone.....it's fixed.

You bought iCloud storage space which has nothing to do with the storage amount on your phone. An iPhone's storage space is a fixed size and cannot be increased. If you need more space, you will need to delete something.

What's the difference between device storage and iCloud storage? - Apple Support

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