I have purchased refurbs on Amazon but they came with a product key sticker so I could complete the activation.
There were some refurbs that used a cracked W10 OS so when you reloaded you might trigger the issue you have now. W10 and all Windows have a license to deal with so in your case you would check your case for the sticker and use that key to activate.
All that said I occasionally run into folk that were ripped off by the refurd seller and even if so, you are not entitled to a free CDKEY.
If you were ripped off, then you talk to the seller.