Max active sites reached error
When a new purchase is made, or when a new account is created for AppSumo users we automatically create one license key for you, so you can immediately start using WP Reset. That key is limited to one site, and has the WP Reset cloud feature enabled. That's why you can use it on only one site. If you try to activate on more sites you get the "Error: Unable to activate site. Maximum allowed number of active sites (1) has been reached." message.
However, if you go to Dashboard - Licenses you can create as many new licenses as you need. Or you can edit the existing one. More info on managing licenses.