Good day Christ1575,
Could you share your subscription that you use to run the FRE and configure the Marketing app?
We met one similar issue that the customer has bought 1 Dynamics 365 Customer Engagement Plan Enterprise Edition license in order to have the Marketing app entitlement(D365_Enterprise_P1).
The problem is that the customer is receiving multiple errors while trying to perform different actions. For instance, while creating a "Marketing Email" and clicking on "Check for errors", the customer received a "MessageContentValidator_PartCode_DefaultContentSettings: MessageContentValidator_MsgCode_DefaultContentSettingsNotSet"
The issue occurred because they aren't eligible for the Marketing app with only a single Enterprise license, so therefore this is considered unsupported until they have 10 or more licenses or have purchased the stand-alone app.
Regards
Johnny