CSV Import>Getting Error Invalid Lead Source Reference Key

When importing Entity Records that includes assigning Lead Sources fails and the CSV Response file throws an error: Invalid leadsource reference key

Common causes of this error:

1. The values on the CSV file does not match the values on the Lead Sources List, e.g. typographical errors
 
2. Lead Source value in the CSV file does not exist in the Lead Source list.

• If Marketing Automation is enabled, the list can be found via: Lists> Marketing> Marketing Campaigns. To add a new Lead Source, create a campaign.
• If Marketing Automation is disabled, navigate to Setup> Sales> CRM Lists> New then click Lead Source to add a new Lead Source.

3. The Lead Source value in the CSV file does not match the naming convention in the Lead Source List.  If the Lead Source Name contains the Campaign ID + Title, the CSV file must have the same value.
    For example: 
        On the Lead Source List: 
                1 LeadSource  
               34 OnlineForm