You can activate it manually?
Just search "Activate windows" in the start menu. If your key is legitimate, it should activate. If you're certain you're in the right and you're still not having luck, you can try running (as admin elevated command prompt, cmd) SLUI.exe, 4
This will call up the phone activation dialog and I've found it to be fairly liberal.
If you bought it on one of those grey market sites, well.... now you know why it was so cheap. They often resell keys in violation of the licenses and ones that are stolen and cancelled when the card owner charges back. These fail fairly often, but the marketplace has little to no liability for this, hence the price. In future, if this is the case, don't buy from sites like Kinguin or G2A.