Bummer.
Sounds like it's either not allowed to access that file path, or somehow the path syntax being used is wrong and access is denied because the path is invalid.
Maybe try to instruct Sonos to play a file that it already knows how to play (i.e. a music file from a folder it already accesses) to make sure you're getting the path syntax correct. That is, in your VC command, try to get it to play that file.
If that works, then try copying the TTS wav to the same folder on the music server (or wherever it is) and see if it will play the wav in that location.
If that works, then either get VC to save your TTS to that location when it's being generated, or try to figure out the difference access-wise between the original path and the path that works. Maybe Sonos can only play files from paths that have already been registered with it as valid sources, or something along those lines.