Have you deleted files and apps to clear the storage? The phone will require a certain amount of memory in order to factory reset.
Did you recently delete pictures or videos. Hint: they don't delete from your device, they get stored in the thrash bin in gallery and take up a lot of space.
Than go to device care and run it.
Than check for any unused apps. If you don't need it, or don't use it... it goes bye bye. Than you can factory reset if you need too.
Have you tried clearing the cache partition on your phone?