I can not find the tvheadend softy? version Armbian_5.38_Bananapi_Debian_stretch_next_4.14.14.7z

It is enabled for Ubuntu Only. I guess it (used to?) fail on Debian.
https://github.com/armbian/config/blob/dev/softy#L25

 

Check manual way: https://github.com/armbian/config/blob/dev/softy#L464-L475

I'll add back if it works.

https://tvheadend.org/projects/tvheadend/wiki/AptRepository

Nope, it doesn't work on Stretch. Method for Raspbian Stretch also fails with: "doesn't support architecture 'arm64'"

 

You probably need to build from sources then: https://github.com/tvheadend/tvheadend or use Ubuntu Xenial, where it works out of the box.

Yes, I use the Ubuntu xenial version on my Cubietruck with Armbian Debian stretch

deb https://dl.bintray.com/tvheadend/deb xenial unstable
# deb-src https://dl.bintray.com/tvheadend/deb stretch stable
#4 components are available:
# release - for the latest official release
# stable - for nightly build of latest release branch (will pick up fixes quicker than release)
# unstable - for nightly build of master branch (don't use on production systems)
# obsolete - historic 3.4 build to keep older users happy

However you have to be aware that the last time I looked not all "components" have available arm binaries