This message will be shown by the Zoolz desktop client in the following two cases:
Case 1: You are a Home user
Home users are not allowed to backup Windows servers. This feature is available only for the Business subscribers.
For more information about the differences between the Home and the Business subscriptions, please check this comparison.
Case 2: You are a Business user, but you have used all the available licenses to backup Windows servers
In this case, you will need to upgrade your plan to a new one with more server licenses. You may contact the Sales (firstname.lastname@example.org) for help with this matter.