While AutoMerging, you may run into this error if you’ve removed fields from the entity you are attempting to AutoMerge.
During the AutoMerge of a duplicate set, fields from the Losing record(s) are preserved to the Winning record if the field is null (empty) on the Winner and populated on the Loser. The list of fields to preserve is defined in an AutoMerge Configuration record within your CRM system.
Go to “Advanced Find” and search for all active AutoMerge Configuration records. There will only be 15 or so records returned. Find the record for the appropriate entity (e.g. Contact) with the “Configuration Type” set to “Preservation Field List.”
The Configuration Value must have a field in it that you have recently removed from your CRM. That is what is causing the error during AutoMerge. you can remove it manually OR use this trick to refresh the list of fields automatically:
Notice that the word all you entered is replaced by a refreshed list of all metadata fields from that entity definition.
Click “Save” then “Close” and validate you don’t get an error during AutoMerge operation.