Hi,
The application can run on High Sierra, however, as with recent versions of macOS, you will have to set "Allow apps downloaded from:" to "Anywhere" in "Security & Privacy" in the Settings.
I believe in High Sierra you will require an extra step as you may notice that the "Anywhere" option is hidden.
You will need to open a terminal window, then type "sudo spctl --master-disable", press enter and type in your password. You should now see the "Anywhere" option after opening "Security & Privacy" again.
I apologize for the inconvenience -- in the upcoming version of Repetitions, the Mac version will be signed as well as be available in the App Store so that there is no need for the steps above.
Best Regards,
Repetitions Support