I need help buying more storage

Hi, welcome to the Apple Users Community.

https://support.apple.com/en-us/HT201318

This will help you buy more iCloud storage.

If your phone is full, see this in settings

you may need to try deleting old text messages and backing off photos/videos to another device for storage to free up space.

Define “not letting me”. Exactly what happens when you try to make the purchase?