NOTE that this error will only come if you are using the LEGACY version of SignAgent Pro. SignAgent is no longer providing updates to this version, as all development has been switched to the cloud version of SignAgent.
To book a demo of the new version, schedule a time here: https://signagent.as.me/demo
There are 2 possible reasons you could be receiving this error:
The last version of the SignAgent scripts are only compatible with InDesign CC 2018. If you're using a newer version, you may get this error.
If you're using the correct version, you may need to re-install the ExtendExtendScript Installer. You can find instructions on how to install the scripts, including this Installer, here: Install Scripts