I had the same issue with iPhone 7 Plus but this is what fixed my solution, if you have tons and tons of photos in your gallery DELETE THEM to free storage, I deleted roughly 3000 (yikes I know). Then after the process of deleting apps or anything taking up storage check and nothing still shows then check if you’ve downloaded any 3rd parités or have signed up for any beta REMOVE IT and you should get the new ios14 update asap. If that doesn’t help Idk what to say, your iPhone may not be compatible 6s+ and up are the only ones I know of that are compatible.