I bought iCloud storage but my iPhone doesn't have enough space

It's probably your iPhone that's out of space, not iCloud. Check this What's the difference between device storage and iCloud storage? - Apple Support

Try freeing up some space on your phone

Manage your photo and video storage - Apple Support

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

https://www.macrumors.com/guide/clear-storage-space-iphone-ipad/

You purchased iCloud storage. You can't increase the storage on your device. If you need some storage you'll need to delete some apps, pictures, videos, etc.