Using Workfront outlook plug-in shared mail box

Have you installed the plug-in for that particular in-box?

https://experienceleague.adobe.com/docs/workfront/using/adobe-workfront-integrations/workfront-for-outlook/set-up-workfront-for-outlook.html?lang=en

The Outlook Connector must be authorized to read the contents of the mailbox, and the process through which that authorization is granted doesn't appear to support shared accounts.

The process is called OAuth, which includes a "scope" rule. Scopes are a list of the actions that the connector/app is allowed to perform - like "read the contents of a user's email" or "send an email on behalf of the user." The scope of the Workfront Outlook connector only applies to a user's account and does not extend to other accounts that are shared with the user.

The fastest way to use the connector on messages in a shared account in Outlook is to drag and drop the message from the shared account to a folder in your user account. This will create a copy of the message in your user account that will work with the connector. You might consider setting up a special folder for that purpose and putting it in your Outlook favorites.