So first we need to clarify what does what and where your actual issue is.
Sentry does not record video- Dashcam does.
Are you able to activate the dashcam feature and is it recording video? If not, your issue is unrelated to sentry at all- you have a dashcam problem (which could be as simple as you just don't have dashcam turned on to record).
The only thing sentry does, as far as video footage goes, is if it goes to an alert state, and dashcam recordings exist, it will move the previous 10 minutes from the recent folder to the sentry folder.
So if you DO have dashcam/recent clips, but DO NOT get anything in the sentry folder when sentry alerts trigger, then and only then, is it a sentry problem.