Tvheadend

It is probably a problem of privileges; the tvheadend process is probably not able to write to the tvheadend-testing directory in the @appstore directory of the Synology.

On my Synology DS213j, the tvheadend-testing directory belongs to the user #166568, defined on my Synology as user "sc-tvheadend".

If it really is a problem of write privileges it is not a problem in tvheadend; it is a problem in the tvheadend package for the Synology. You should try to contact the person that created the package to have it fixed. (I remember also having such a problem and fixed it manually on the Synology by giving the tvheadend-testing directory the appropriate privileges.)

It might be related to the following issue of the framework to cross compile packages for the Synology:
https://github.com/SynoCommunity/spksrc/issues/2216