You can only register the app group to workspaces created in the same location as the host pool. Also. if you've previously registered another app group from the same host pool as your new app group to a workspace, it will be selected and you can't edit it. All app groups from a host pool must be registered to the same workspace.
Reference:
https://docs.microsoft.com/en-us/azure/virtual-desktop/manage-app-groups
