The most common cause of this error is due to the deployed status of this field within the LCRM package. Please ensure that it is deployed by following these steps:
  1. Login to LCRM
  2. Navigate to Setup, Create, Objects
  3. Find "Related Email Address" object
  4. Click Edit
  5. Scroll down to Deployed status and select "Deployed"
  6. Click Save
You should now be able to merge contact records without getting this error.