This error means that your EKG stopped recording before the 30 seconds required to be processed by AliveCor's algorithms. This can be caused by letting go of the electrodes on purpose or accidentally by trembling or moving.
At this point you have the option to save the EKG you did record with no algorithm processing, or tap "Record a new EKG" to start a new recording.
You may also want to read this article for some pointers: How do I get a clean EKG signal with KardiaMobile?