Description: This solution describes how to resolve the error Update_Source is Missing or Does Not Have a Valid Value
This error occurs because update_source is required when updating a contact in Constant Contact and only accepts values of "Account" or "Contact". Because the contact is being updated through the Admin side via the API, the value to use is "Account".
In your formula, go to Data Mapping.
For the Update Source field, select Defined from the drop down and type Account
After saving your changes the formula will reprocess and you can send the record(s) through to Constant Contact.