REASON:
Snap Captions Tries To Generate Captions but there isn’t a timeline opened.
SOLUTION:
You can open up a timeline to get Snap Captions to work. At the time of generating the captions, your timeline should be active (open) and look something like this:
If you are only trying to install Snap Captions, or check if it has installed correctly and get this error, you can open an empty timeline.