You can download and install Windows 10 Pro for free from official Microsoft download page:
https://www.microsoft.com/en-us/software-download/windows10
How to perform a clean installation: https://www.howtogeek.com/224342/how-to-clean-install-windows-10/
And after installing Windows 10 Professional follow this:
1.Go to Settings
2.Select Update and Security
3.Go to Activation
4.Click on Activate or Change Product Key
5.Enter the License Key you received after purchase
6.Done your Windows is now fully activated
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If you don't want to do a fresh installation, then please follow this video tutorial to upgrade from Windows 10 Home to Pro and then use your product key.
1.Follow this video tutorial:
https://www.youtube.com/watch?v=F_jGe1kRBZo&feature=youtu.be
2.Then activate the Windows with your Product Key
Note: this solution is only if you have Windows 10 Home, if you still receive any error during the upgrade process, then new installation is must.