Have you ever searched forums a night long just to find stupid answers like “Did you restart, check your drive letter, etc.” because any OS would tell you that it cannot write to or format your USB key because the permission was denied? If yes, here’s a solution you may have never thought of:

Check on the physical body of your USB key, maybe there’s a write protection switch turned on. Yes, as SD cards do, some USB keys are uselessly equipped with that hardware feature. And by uselessly, I mean that it causes more problems than it solves them because it can become loose, or you could turn it on by accident.