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
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.