Can I purchase Web Pro edition license and run it on WebHost AWS instance? For example, if I have Plesk Onyx on Ubuntu 16.04 (WebHost).
With instance that already includes Plesk license you cannot use any other Plesk edition.
You should order a new instance, which called “Bring Your Own License" (BYOL). Such instances do not include Plesk licenses and you can purchase license for any preferred edition of Plesk.