When you try to activate Windows 10, you will see 0x800f0805 error
This error appears when you buy the wrong key for your Windows 10. I example you use Windows 10 Home but you buy Windows 10 Pro key.
So if you want to fix 0x800f0805 windows activation, you upgrade your Windows 10 to the right edition by using Windows 10 upgrade key
If you use Windows 10 Home, you can upgrade to Windows 10 Pro easily. You read this article: https://greenluffa.com/how-to-upgrade-windows-10-home-to-pro/
If you use Windows 10 Pro/Windows 10 Edu/Windows 10 Enterprise, you can change it to another edition easily. You read this article: https://appsforpcfree.net/upgrade-windows-10-pro-enterprise-education/