I've seen this error before when connecting to a shared mailbox, as the account I was authenticating the connection with only had read and access permissions on the mailbox.
Has the account that you've authenticated the connection with got all of the permissions it needs in Azure AD? The following permissions are required according to the docs:
https://outlook.office.com/mail.read
https://outlook.office.com/mail.send
https://outlook.office.com/calendars.readwrite
offline_access
https://outlook.office.com/mail.Read.Shared
Workato connectors - Outlook | Workato Docs
