This field can be empty depending on user input.

See How to Resolve Configuration Errors

In your case, the your-subject field is not a required field, so it can be empty if submitter left it blank.

OK,
Thanks; but in “How to resolve…” I read:

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.

So how can I make the subject field a required field?

See https://contactform7.com/text-fields/

I guess you use text now. Use text* instead.