I can’t purchase more storage as I’m trying to do so

You can't purchase more storage in an iPhone, it is a fixed size. You can purchase more iCloud storage if that's what you mean.

The difference between phone storage and iCloud storage -

https://support.apple.com/en-us/HT206504

Upgrade your iCloud storage plan -

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

Freeing up storage space on your iPhone.

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, iPad, and iPod touch

https://support.apple.com/en-us/HT201656

Manage your photo and video storage

https://support.apple.com/en-us/HT205703

Clear Other storage on your iPhone or iPad

https://support.apple.com/en-us/HT211686