The Avast VPN license number for Windows PC OS is different to the iMac OS, because, about a month ago a friend of mine couldn't use the Avast license from Windows PC OS platform to iMac OS platform because of the Avast license coding platform. As Asyn replied you need to contact the Avast support to give you a new license code from Windows PC OS to iMac OS platform.
I hoping Avast would allow these licensing platform to work using the same license switching from Windows PC OS to iMac OS.