Remove the battery from the back of the Pendant and then wipe off both the battery and the Pendant
Clean the gold contacts with a little rubbing alcohol or hand sanitizer for best results
Put the battery back on the Pendant and secure using the battery latch at the bottom.

Place finger-tips inside the cradle and firmly press the Patch against the skin; then move fingertips all around the edges of the Patch to make sure the Patch is secure against the skin - no pockets of air between the Patch and skin.
Put the pendant back in the cradle and verify that the error message disappears