My device has 21 gb free space and still it’s showing memory full

You sure it's the phone's local storage that is low, or is it possibly telling you that the iCloud storage is low? They are two separate things: https://support.apple.com/en-us/HT206504. If it is the iCloud storage, you can manage it with https://support.apple.com/en-us/HT204247.

I'd suggest that you back up your phone to your computer and then restore it from the back up. That will generally fix any issues with corrupted data that result in incorrect storage space readings.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support