message of iPhone storage full with 20 Gb free

I agree, it looks OK to me as well - assuming you keep few if any photos in iCloud Photos.

Well, I’m perplexed. If this happened to me, I would first update to the current iOS version (15.5). Update your iPhone, iPad, or iPod touch - Apple Support

If that doesn’t either solve the issue or shed more light on the underlying cause, I would visit a Genius Bar and see what they have to say.

Are you certain the message refers to your iPhone storage? Might it refer to iCloud storage?

What's the difference between device storage and iCloud storage? - Apple Support

You should see more details when you tap here.

OK, I understand, thank you.

What do you see at Settings > [your Apple ID] > iCloud > iCloud Storage? Do things look normal there? Do you see any spinning circles?

Good luck at the Genius Bar! It may well help other folks if you post the results of your visit here, if you don’t mind. Thanks.