@BLUEFROG worked through this with me on a support ticket.
The issue is that DT3 needs to be on this Security menu:
The problem is that if you inadvertently answered “No” the first time it asked for permission, there is no realistic way to fix that short of re-installing the whole OS from scratch or some other similar major redeployment of the operating system.
I settled on a workaround whereby I call the script through a Keyboard Maestro macro. The script works fine if it is initially called external to DT3.