Why I can’t buy iPhone storage

First of all, there is NO way to add memory to an iPhone. Once purchased a phone's physical memory is fixed at purchase.

If you need to buy iCloud Storage to be able to sync more data with iCloud, this link will tell you how you can do that, but it will NEVER add space to your phone at all.

If you want to buy more iCloud storage, click here --> Upgrade your iCloud storage plan - Apple Support

If you are low on memory on your phone, you will need to delete things from your phone, such as Apps, Photos, Music, Messages, Email, Files, etc.