Unable to transfer Windows License to new computer

I have an old HP PC that came with Windows Vista, but right now it runs Windows 10 Home, and is activated with a valid license. As the old PC was unsupported for Windows 11, I built a new custom PC and installed Windows 11 Home from an installation USB, and I would like to transfer the license from the old HP PC to the new custom built one.

I signed in with my Microsoft account to link the old PC to my account. Settings shows that "Windows is activated using a digital license linked to your Microsoft account". I believe the license is Retail, because I ran the command `slmgr /dlv` in elevated Command Prompt, and it showed the Product Key Channel as 'Retail', and the partial product key as the last 5 digits of the generic key.

I signed in with the same account on the new PC, and on the activation page it said Windows is not activated. So I clicked activate now, then clicked troubleshoot, then "I changed hardware on this device recently", and selected the old PC, but I get the message: "We can't activate your copy of Windows because the digital license for this device has already been used to reactivate Windows."

I also called Microsoft Support, and they said that the licence couldn't be transferred, but I didn't fully understand why, even though slmgr shows the licence as Retail.

Is there any way I can fix this to transfer the licence without having to buy another one?