You can manually create the access file if you really have to...
$ sudo su hts
[sudo] password for <you>:
$ cd ~/.hts/tvheadend
$ vi superuser
{
"username": "blob",
"password": "blob"
}
:wq
... and then restart tvheadend:
sudo service tvheadend restart
You should now be able to log in as blob/blob, and then configure access control etc. as required.