The Marketo sync user needs to be a Marketing user for the Salesforce campaign sync to work properly. Here is how to make the user a Marketing user in Salesforce.
Admin Permissions Required
Log in to Salesforce. Search for users in the left search bar and click Users under Managing Users.
Find your sync user and click their name.
Check the Marketing User check box and click Save.
Great! This Marketo sync user is now a marketing user.