As far as Windows10 is concerned – I’m not sure why you didn’t just get it directly from Microsoft to begin with (you can download the image, plus the media creation tool for free).
As far as the getting a key, you can try KMSpico like many of us had preinstalled, or ebay, or even https://www.reddit.com/r/microsoftsoftwareswap.
I’m unfamiliar with the problem you’re having, but if I were in your shoes, I’d download a fresh Win10 image from here https://www.microsoft.com/en-us/software-download/windows10 – then use the media creation tool to find a means to install it to your tablet. Once windows is running, I’d then use one of the three options I listed above to “validate” that copy of windows.
Hope that helps, and good luck!
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