Why I’m not able to buy storage on my phone

You cannot increase storage of iPhone. However you can optimise iPhone Storage by using iCloud Storage. Save space on your device

Settings > [your name] > iCloud Tap Photos > Choose Optimize [device] Storage Set up and use iCloud Photos - Apple Support

You can buy iCloud Storage. --> Buy more iCloud storage - Apple Support

You can manage your storage --> How to check the storage on your iPhone, iPad, and iPod touch

Manage your Storage --> Manage your photo and video storage - Apple Support

Your photos are always available on both iPhone and iCloud. You delete it on iPhone it is gone from iCloud and vice-versa. To save space on your iPhone use Optimised iPhone storage. Remember, if you delete in one it is deleted everywhere.

Get started with iCloud Photos:

1. Tap Settings > [your name] > iCloud > Photos.

1. Turn on iCloud Photos.

1. Select Optimize iPhone Storage to save space on your device.

With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. And as long as you have enough space in iCloud, you can store as many photos and videos as you want.

How to check the storage on your iPhone, iPad, and iPod touch

Because the phones are designed with the memory that is available to them when they are built. The amount of storage memory that is installed in them is all they will ever be able to use. It’s the hardware that is installed INSIDE the phone, and you can’t just go and put more inside it. It’s soldered to the main board and what’s in there is what’s available. You should always consider what you’re going to need at the time you purchase your phone and always go with more then you think you’ll need. It’s always better to go over then you don’t have to worry about using too much.