Please check first if you can open the web interface of the TVheadend receiver:

Find out the IP address of the TVheadend server (the IP address should be static and not change)
Open the web interface e.g. at http://192.168.0.1:9981 (enter IP address)
If you have activated the option "Digest" for Authentication Type in the Configuration - General - Base - HTTP Server Settings area, activate either Plain or Digest and Plain. Otherwise the setup will work, but streaming will not.

Also please make sure you assigned channels to tags because the app uses tags to group channels. See TVheadend help.