In order for an acknowledgment to be generated in Salesforce, the Receipt Contact field must be populated.  

• If you populate the Primary Contact field when first creating the opportunity, it will automatically add that person to the Receipt Contact field. Otherwise, the Receipt Contact must be added manually.
• Whoever is populated in that field must have an address on their contact record. The address will not be used in the acknowledgment letter, but it needs to be populated or the flow will not trigger in Salesforce. If you don't have the contact's address, you can simply enter the address of their Primary Affiliation.
• The Primary Contact and the Receipt Contact might not be the same person but they can be. :)