This error (and similar) generally means that something went wrong with the original installation and a required component of eSellerate (our licensing and activation provider) didn't install properly. Please download and install the eSellerate Engine Update and try activating again.
If this doesn't work (macOS-only), try deleting the /Library/Frameworks/EWSMac.framework and /Library/Frameworks/EWSMac-GC.framework folders, and then install the eSellerate Engine Update again.