When you purchase a license key for a software solution (i.e- RollBack Rx, Drive Vaccine or Drive Cloner)  from Horizon DataSys, there is no limit to the number of times that license key may be activated - these licenses are perpectual as well.

Single license key can only be used on a single machine at any given time. 

When you use the license key for the first time it will "lock" itself to that specific computer. If you subsequently do a proper uninstallation of the software from that PC, your license key will automatically be "freed" up for you to reuse.

If you own a single Product ID on which you have multiple license keys then a single key can't be isolated and reset. Instead that Product ID  will be reset. All the machines on which you have the software installed with that Product ID will initiate an activation check. If they're connected to the internet this will occur in the background and you won't even notice this automatic re-activation. Any of your licenses key that are not currently "locked" on any machines will then be "freed" up and are available for reuse.

In order to resolve this error:  Please "submit a ticket" with that Product ID and your email address that you used for the purchase.