[your-subject]- error

See How to Resolve Configuration Errors

But what does that mean? I am allowed to use [your-subject]. But when I do, I get an error message. And please don’t point me to your How to solve-site, I don’t understand your solution there.

The subject is not allowed to be blank. If you use mail-tags here, ensure that it’s not blank even after the mail-tags are replaced with user inputs.

Imagine if the user does not include a subject in the form. In that case, the subject line will still be blank even with [your-subject] inserted. Consider using an * to make the subject a mandatory field to complete.