Previewing is possible only when you have the first section and last section of the video file, look at the piece map and see which pieces you have. And not all players support incomplete files. I suggest you use either MPCStar or VLC.
As Vasy suggests, the problem with previewing may be that you don’t have the first, last, and enough other content of the file to preview. When downloading a video file and preview mode is enabled, bitcomet will download the first and last piece first. If the torrent contains multiple files, make sure you don’t alter their preferences. If you have one or more of the files disabled so it doesn’t download and then change it later so it does download, then bitcomet won’t be able to properly calculate where the first and last piece are. In such a case you may have to stop the task and perform a manual hash check on the files.
Please also understand that preview of a partial download isn’t something that is always successful, at best it will be only partially successful, and at worst, not at all.