This error means that your recording was completed, our algorithms processed it and determined that the result is Unreadable as an EKG. This is often caused by signal noise or incorrect recording technique.
You have two options at this point. You can tap "Save this EKG" to save the recording regardless of the result, or you can choose to "Record a new EKG" to try again and perhaps implement some recording strategies.
Read the article What can I do to avoid the Unreadable results? to learn some strategies to avoid Unreadable results such as removing charger cables, sitting still, and moistening your fingers.
You can read this article to Learn more about a 'Unreadable' EKG Result.