Message field in "Scheduler" not accessible

Message field in "Scheduler" not accessible

I am trying to create a schedule for one of my forms; specifically, trying to program the app to send out an email alert one week before the driver's license of the company vehicles' drivers expires.
I run through the "When to do" but when I get to the "what to do" I can only get as far as inserting the message subject.
The field for the actual message is visible but not accessible. I cannot type on it and it does not show the tabs shown in the "Message Subject" field (Insert Expression, Switch to...). It is as if it is locked out.
Zoho Support is still working on this. Am I doing something wrong?
 
Thanks