Posting this solution in case anyone else counters the error:
I set up the SendGrid function and was able to send plain text email successfully using the demo.
I tried using the SendGrid function to send an email using a dynamic template I have set up in SendGrid. When I copied the template ID over, it said it wasn’t a valid GUID. When I removed “d-” from the beginning of the id, it then said that it couldn’t send legacy templates. I’m 90% sure though that this isn’t a legacy template–I created it within the past couple months and it works fine when we ping it from our own server.
Then I realize there was an extra space at the end of the id. Removed that, added “d-” back at the beginning, and then it worked.