Sounds like you've filled up the memory card.
You either need to connect your computer to the scanner in Mass Storage Mode and either delete or move to your computer some of the recordings in \BCDx36HP\audio\user\, or manually go through your recordings via the Menu feature on the scanner (Menu>Settings>Replay Options>Review Recordings) and select them one at a time by pressing E on each date/time entry, then scroll to "Delete Session", and hit E again (and again at the yes/no prompt).
If you want to keep them, connect the scanner to your computer, and move the files in the folder I mentioned above to your computer using Windows Explorer (not in Sentinel).