Salesforce integration- fields not getting pulled through

Hello, I am attempting to connect our Salesforce org to our Constant Contact account. We set up a new user to use for the API integration, with a profile type of Salesforce API Only System Integrations, but when that account gets connected I receive the error: "Could not get Salesforce Fields. You may need to upgrade your Salesforce account." Can someone please advise on how to get this connection working properly? Thanks,