Thank you for posting this in Microsoft Q&A.
I understand you are trying to update your Microsoft Entra gallery application to add new functionality, but when attempting to access the Microsoft Application Network portal, you encounter the error: "Selected user account does not exist in tenant 'Microsoft' and cannot access the application '00000003-0000-0ff1-ce00-000000000000' in that tenant. The account needs to be added as an external user in the tenant first. Please use a different account."
As mentioned in the document you shared, the acceptance of new SSO and provisioning requests for publishing applications in the Microsoft Entra application gallery has been temporarily stopped.
This is why you are unable to submit your application to the Microsoft Application Network portal. You will be able to submit your application once the requests to add new functionality for applications are resumed.
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.
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