Using the terraform auth0 email template using liquid syntax common variables, as the doc suggests and the auth0 form provides returns a 400.
Does the terraform provider only surface the error response status code or do you mean that it failed and the response did NOT include a body with any additional details. If there’s no response body or the body is not surfaced by the tooling this can be a bit more complex to troubleshoot.
Is it possible for you to share the tenant name where this happens? This could allow us to check service logs; if you’re wary of sharing a production tenant name you can always create a temporary tenant just to reproduce this situation and share that one.
Hey there everyone! 
I thought I’m gonna chime in again with something that might be of your interest! We’re hosting an Ask Me Anything Session in our Forum regarding Auth0 Terraform Provider.
It’s gonna be on Thursday, September 28, 2023. Check out more info about it here!