A serial number (called Machine ID) is assigned to each computer. Your Machine ID is already activated, but with a different license key. If you have another license key, try to activate with it. If not, please create a support ticket to get help resolving this issue.
MobileTrust® users can manage their profiles (change password and email address; manage devices; view license key and subscription detail, etc.) using the SFT Cloud Website. Login to the SFT Cloud Website using the same email address and password you use for the MobileTrust® app.