I thought Windows 11 for Mac was a preview so perhaps you cannot buy a licence for a preview build. Generally though Windows 11 licences are not available to buy currently and only devices with it pre-installed are available.
You might have to wait to buy a Windows 11 licence.
As with Windows 10 though if you have Home edition you must activate that prior to buying an upgrade licence for Windows 10 Pro. If you want to have Windows 11 Pro edition perhaps you should install that edition then look at a Pro licence, when available.
Please see the advise from Sumit.