The Salesforce Integration User License and its associated profile, the Salesforce API Only System Integrations profile, are not compatible for use with the Salesforce System User for Marketing Cloud Connect. This is because the MCC Salesforce System User requires permissions to interact with the UI, which the Salesforce Integration User License does not allow. Attempting to use this license for the MCC Salesforce System User will result in errors and loss of functionality.
The recommend license and profile to use would be the Salesforce license with the System Administrator profile as documented in the Marketing Cloud Connect Setup articles.
Marketing Cloud Connect User Types and Permissions
https://help.salesforce.com/s/articleView?id=sf.mc_co_user_types_and_permissions.htm&type=5
Set Up Salesforce System User in Marketing Cloud Connect
https://help.salesforce.com/s/articleView?id=sf.mc_co_set_up_salesforce_system_user.htm&type=5
Give Integration Users API Only Access
https://help.salesforce.com/s/articleView?id=sf.integration_user.htm&type=5
000395036

We use three kinds of cookies on our websites: required, functional, and advertising. You can choose whether functional and advertising cookies apply. Click on the different cookie categories to find out more about each category and to change the default settings.
Privacy Statement
Required cookies are necessary for basic website functionality. Some examples include: session cookies needed to transmit the website, authentication cookies, and security cookies.
Functional cookies enhance functions, performance, and services on the website. Some examples include: cookies used to analyze site traffic, cookies used for market research, and cookies used to display advertising that is not directed to a particular individual.
Advertising cookies track activity across websites in order to understand a viewer’s interests, and direct them specific marketing. Some examples include: cookies used for remarketing, or interest-based advertising.