"Failed to install" isn't really helpful, is it...
Architectures supported here:
https://synocommunity.com/package/tvheadend
Your architecture here:
http://forum.synology.com/wiki/index.php/What_kind_of_CPU_does_my_NAS_have
Looks like the 211j and the 210j have the same CPU, though, so something else must be tripping it over. Is there anything in /var/log/syslog.log, /var/log/messages, /var/log/synolog/* or similar?
My Syno box is long since out of action for this kind of thing, so it may be that you need to follow it up on the Synology/synocommunity forums unless someone has a bright idea here.
(As an aside, I found some reference to a trust level that needs to be set to enable third-party installation as of DSM 5.1 - Package Centre -> Settings. Have you enabled that for non-Synology packages?)