what is sensible-mda and do i need it?

Perhaps you've installed sendmail by using the sendmail-bin individual package instead of installing the sendmail wrapper package.

Anyway, if you install sensible-mda (or the sendmail wrapper package), the problem you're seeing should disappear.

The sensible-* programs are installed so that other system scripts etc. can rely on a program which provides a certain functionality being in the right location.

sensible-mda is a dependency of sendmail, so it should exist on your system.

Also you should seriously consider not running sendmail.

Either exim4 or postfix are much better options for real mail servers, and ssmtp and nullmailer are much better options for hosts that will only send outbound mail.

As this point the only people who should be running sendmail are those with large old installations or a need for some of the very complex non-SMTP routing that sendmail can do.