When using deb package installer make user/group etc
for situation when there is no user but service started,
see with user and group start it usually
tvh:video
stop service and run tvh via command line
tvheadend -u tvh -g video -s -c /home/hts/.hts/tvheadend/
or test with superuser hack
nano /home/hts/.hts/tvheadend/superuser
{
"username": "USERNAME,
"password": "PASSWORD"
}