This is because your tenant lacks a valid Office 365 Business subscription. If you are trying to access business-level O365 services (such as Exchange Online, Outlook, Teams, etc.), then an Office 365 license will be required.
Please follow this document to subscribe to an Office 365 license for your tenant or start a free trial. After that, you also need to grant the license to the target user you are reading/sending emails on behalf of, which will automatically assign an EXO mailbox to the target user.
Hope this helps.
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