DokuWiki expects to be run on a properly configured PHP setup. That entails a proper setup for mail sending. On Linux Systems PHP relies on the operating system to have a correct mail setup, eg. a local MTA with a sendmail compatible binary to push mails into the local mail queue. How to setup an MTA is a bit beyond the scope of any DokuWiki setup guide. Usually on Debian it's enough to install the appropriate package and answer the questions asked by the dpkg setup process. I believe Debians default MTA is Exim, personally I always found Postfix easier to setup. You should definitely not bother with a real sendmail install.
Alternatively you can also install the smtp plugin for DokuWiki and setup an external SMTP server for sending mail.