This issue can occur if your Exchange mailbox was recently setup but has not yet been migrated from Exchange on-prem to Exchange Online. Newly created mailboxes are usually migrated within a couple of days.
Contact the Service Desk or your department IT. We can confirm whether your mailbox has been setup successfully in Exchange Online. It is best to wait until the mailbox has been setup on Exchange Online before configuring mobile apps.
Until your mailbox is setup, you can access your email via a web browser at: https://exmail.oregonstate.edu/owa