"You are updating data but have not defined a key" message

QUESTION:

I am receiving the error "You are updating data but have not defined a key" when trying to update/upsert/insert data in MassEffect.

ANSWER:

There a two possible causes of this error: 

1. You are attempting to update existing records but have not mapped a column from the input file to the "key" field on the table. This is typically the Salesforce "id" field shown as the first field in the list of fields with a key symbol. This is the unique key to identify which records should be updated. 
2. You are attempting to insert new records, but have update selected as the operation on the right (which is why it is looking for something to be mapped to the key field to identify which records should be updated). 

Change the operation on the right to insert then remap you fields.