Why my phone keep saying no space

Settings, general, iphone storage

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Consider iCloud Photos and Optimise Storage options.

Until you shift data from the phone to iCloud you will not gain from your purchase.

Manage your iCloud storage – Apple Support (UK)

and

What’s the difference between device storage and iCloud storage? – Apple Support (UK)

Buying iCloud storage doesn't help.

You don't get more space on your iPhone that way.

The amount of GB's your iPhone came with when you bought it is all you have.

So better manage it!