A Configuration Error that Should Be Fine

I’m being told I have one configuration error. I looked at the back end and found that under the mail tab it’s highlighting my subject field which I have populated with [your-subject]. But, [your-subject] is what I’m told to use. The field is highlighted in red and it says, “This field can be empty depending on user input.”

Is there something I’m doing wrong? Why is this being tagged as a config error?