Since Apple has released a new update for Mac OS 10.14 Mojave, some users are facing difficulties to scan text outside the IRISPen application.
The good news is that fix is really fast and easy.
You have to go to System Preferences > Security & Privacy > Privacy tab :
Once you are there, click on the lock on the bottom left side and enter your Apple ID password. Go to Accessibility and check the IRISPen line.