This error is shown when your KardiaMobile 6L's battery is too drained to take any more recordings. This error should have been preceded by an alert of low battery, and a low battery indicator on the recording screen over time.
When this error is seen you can no longer record and need to change your battery as outlined in this article: How do I change the battery in the KardiaMobile 6L?.
Typical battery life of the KardiaMobile 6L is 1-2 years with typical use.
Note that this ONLY applies to the KardiaMobile 6L. KardiaMobile and KardiaBand do not have any battery level indicators or errors in the Kardia app.