Most likely a compatibility issue with DSM 6.0. You can take a look at the SynoCommunity / spksrc forum / github to get an idea about how much "fun" the 6.0 upgrade has created for the 3rd-party package community, but J. Dierkse works independently so the SynoCommunity folks won't support his testing packages or answer questions in that regard...
I see three options for you (might miss others):
(1) You downgrade your Syno to DSM 5.2, if you are still in early stages of setting it up, or you really, really want TVH-Testing
(2) You hope somebody has found some workaround or will be finding one soon (not aware, but haven't looked and might be!)
(3) You hope, watch and wait until J. Dierkse has fixed the issue. TVH-Testing seems to be his private fun project, but haven't seen him in any forum discussions in recent weeks and months, so you simply never know if and when...
Good luck!