Hi, I'm John!
If your Windows 10 Pro retail license is genuine and currently activated, you normally do not need to purchase a separate Windows 11 Pro key. A Windows 10 Pro retail key can be used to activate Windows 11 Pro.
- Are you upgrading the motherboard, CPU, or the entire PC?
- Is your Windows 10 Pro currently activated?
- Is the retail license linked to your Microsoft account?
If this is a new PC build, you can install Windows 11 Pro using a USB installer and then activate it with your existing retail key. If the license is transferable and no longer in use on another device, activation should work.