Hello Thomas,
Primary concern posted on Q&A: Cannot Associate a Partner Center MPN ID (You are unable to add a verified publisher to this application. Please contact your administrator for assistance)
When it comes to the error message you received, were you able to look at our Troubleshoot publisher verification documentation, specifically UserUnableToAddPublisher.
Note that you can get blocked if your account has been identified as a "risky user." If the user is on the “risky users” list, you or your admin may need to perform remediation steps to eliminate user risk. Reference: Remediate risks and unblock users in Azure AD Identity Protection | Microsoft Learn. After remediating, it will take several hours to invoke dismiss process in our backend, and then you can try adding the MPN ID again.
the issue seems to resolve itself within 24-72hrs. However, you Can try adding your verified publisher MPN using another user (either existing or newly created user) with similar permissions, to see if that helps resolve your issue.
Regarding adding your app to the Entra ID App Gallery, if you're unable to do so, it's possible that there have been some changes to the process. how-to-submit-a-request-to-publish-my-application how to create a documentation for publish an application to Microsoft Entra application gallery Ensure that your app meets the latest requirements for inclusion in the gallery. Sometimes there are specific prerequisites, like having certain permissions or completing additional verification steps. plan-an-application-integration
I hope this clarifies things.