Outlook for Mac users are not able to use the "new" Outlook if they have a shared account in Outlook. For more information, please see this article.
To add a shared mailbox it is necessary to have both of the following:
(i) a security group that grants “Send-as” and full access permissions to all folders of the shared mailbox
(ii) your username must be a member of the mailbox security group as your account requires permission to the shared mailbox for authentication using your own credentials.
Contact IST Service Desk or your computer support rep if you need help to meet the above requirements.