Hi Kelly,
To enable App for Outlook, Server-side synchronization should be configured;
to configure Server-side synchronization, Exchange online is needed and required to included in license.
You could view detailed license items of users who only have marketing license to check whether they have this option, once it exists in license, these users would be able to use too.
In Microsoft 365 admin center, navigate to Users > Active users, edit the users who only have marketing license.


Regards,
Clofly