That's why people prefer HP to Dell. I don't have any problems with upgrading my HP laptops OS to windows 10 so far. Some are pretty old(more than 10 years.)That was a promo for dell laptops in 2015.
youll have to buy a copy of windows 10.
i stand corrected here is a list of the models that support the upgrade to windows 10.
https://www.dell.com/support/kbdoc/en-us/000130221/windows-10-upgrade-walkthrough
I tried to edit the registry but to no avail. It seems that this problem cannot be fixed.Start by fixing the error code. It's there for a reason.
https://www.thewindowsclub.com/fix-0x80072f8f-0x20000-windows-upgrade-error