From the FAQ
https://tvheadend.org/projects/tvheadend/wiki/Faq
:
"Help! I can’t login to the web user interface.
If you build Tvheadend from source and just execute it, it won’t let anyone in. The rationale behind this is that we think it is bad to have services running with wide open administrative access.
If you start Tvheadend with the ‘-C’ argument. A default user with no user-name or password will be created. Make sure that you later edit this via the Configuration / Access control tab in the web ui.
If you start Tvheadedn with the '--noacl' argument, all Access Control will be removed and you can setup required users via Configuration / Access control tab.
If you installed the Tvheadend debian package you should have been asked for a superuser username + password during the install. This super user account can not be modified from within Tvheadend itself. If you want to change it, you need to reconfigure the Debian package. To do that, execute the following line in a command prompt:
sudo dpkg-reconfigure tvheadend"
HTH