Hello everyone!

I've tried so many times I'm getting really tired of this, so if anyone here can help me then I'd be very grateful! (Will buy you a beer! )
In short: the computer would not boot into its original Win 8.1. I created a bootable Win 8.1 usb and tried both a recovery which resulted in an error and 'fresh' install which I couldn't initiate due to licence key incompatibility (I have OEM key and the installer would complain that it can only be used with the OEM version).
Reading somewhere that Win10 can access the UEFI stored licence key, I decided to give it a go. It boots, I can go past the licence agreement and then the installation drive selection.
This is where it stucks - if I select the eMMC it will fail (attempts to format or delete partitions on this also bring no results, even if using gParted or diskpart), I successfully formatted the 500GB HDD but installer prevents from installing the system there (saying it cannot use this type of port).

Any insights?