I wasn't sure since I did most of that yesterday, or early today, so I did for sure this time, similar message after the apt-get update, although a bit different, and the same message after the sudo apt-get install tvheadend.
pi@rasbian-kodi:~ $ sudo apt-get update
Hit:1
http://mirrordirector.raspbian.org/raspbian stretch InRelease
Ign:2
https://dl.bintray.com/mpmc/deb rasbianstretch InRelease
Ign:3
https://dl.bintray.com/mpmc/deb rasbianstretch Release
Ign:4
https://dl.bintray.com/mpmc/deb rasbianstretch/stable all Packages
Ign:5
https://dl.bintray.com/mpmc/deb rasbianstretch/stable armhf Packages
Ign:6
https://dl.bintray.com/mpmc/deb rasbianstretch/stable Translation-en
Ign:4
https://dl.bintray.com/mpmc/deb rasbianstretch/stable all Packages
Ign:5
https://dl.bintray.com/mpmc/deb rasbianstretch/stable armhf Packages
Ign:6
https://dl.bintray.com/mpmc/deb rasbianstretch/stable Translation-en
Ign:4
https://dl.bintray.com/mpmc/deb rasbianstretch/stable all Packages
Ign:5
https://dl.bintray.com/mpmc/deb rasbianstretch/stable armhf Packages
Ign:6
https://dl.bintray.com/mpmc/deb rasbianstretch/stable Translation-en
Ign:4
https://dl.bintray.com/mpmc/deb rasbianstretch/stable all Packages
Ign:5
https://dl.bintray.com/mpmc/deb rasbianstretch/stable armhf Packages
Ign:6
https://dl.bintray.com/mpmc/deb rasbianstretch/stable Translation-en
Ign:4
https://dl.bintray.com/mpmc/deb rasbianstretch/stable all Packages
Ign:5
https://dl.bintray.com/mpmc/deb rasbianstretch/stable armhf Packages
Ign:6
https://dl.bintray.com/mpmc/deb rasbianstretch/stable Translation-en
Ign:4
https://dl.bintray.com/mpmc/deb rasbianstretch/stable all Packages
Err:5
https://dl.bintray.com/mpmc/deb rasbianstretch/stable armhf Packages
404 Not Found
Ign:6
https://dl.bintray.com/mpmc/deb rasbianstretch/stable Translation-en
Hit:7
http://archive.raspberrypi.org/debian stretch InRelease
Reading package lists... Done
W: The repository '
https://dl.bintray.com/mpmc/deb rasbianstretch Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch
https://dl.bintray.com/mpmc/deb/dists/rasbianstretch/stable/binary-armhf/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
pi@rasbian-kodi:~ $ sudo apt-get install tvheadend
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package tvheadend is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'tvheadend' has no installation candidate