Factory reset keeps failing with error 4013

If you’re getting an error trying to reset with iTunes and you have already tried using another USB cable/port/host machine, more than likely this error is a hardware fault in the phone. Error 4013 is less specific than some other errors, but in my experience, using a different app to reset is not likely to fix this issue and may actually cause further issues. Error 4013 specifically is often a logic board related problem

If I recall correctly, on iPhone 6, it can be related to NAND (the device storage), but could also be related to any number of other board level issues. Either way you would likely need someone to diagnose and repair the logic board itself to resolve the error.