I’m trying to buy more iPhone storage but am not given the option on my settings

It is not possible to buy more iPhone storage.

Possibly you are referring to iCloud storage? If so, the following may help: Buy more iCloud storage - Apple Support

Note that iCloud storage can only help with a shortage of storage on an iPhone in a limited way. See the following for more information: About storage on your device and in iCloud - Apple Support

Assuming that you are talking about iCloud storage and if your payment method is valid, or that you have a balance in your Apple ID, if you cannot do this on your phone, explain what happens when you try it.

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage.
2. Tap Buy More Storage or Change Storage Plan.
3. Choose a plan.
4. Tap Buy and follow the onscreen instructions.