It used to track it but always said bad sleep ... now there is nothing!
I would start with restarting your watch: Sometimes, a simple restart can fix the issue. Turn off your watch and turn it back on after a few seconds.
If still an issue, check this support site for possible answers - My Sleep Statistics Are Not Displaying in Garmin Connect | Venu® 3 | Garmin Customer Support
If nothing helps, I recommend reaching out to Product Support through their website at https://support.garmin.com.
