Try to check in Outlook under Settings, Accounts, Email accounts and confirm your account shows no errors. Then go to Settings, General, About Outlook and make sure the app is updated. If the issue continues, reset Outlook from Windows Settings, Apps, Installed apps, Outlook, Advanced options, then select Reset and sign in again.
If it also fails in the browser, contact Microsoft Support through Help, Support, then Contact Support inside Outlook to reach an agent.