We are aware of an issue impacting On-Premise users being unable to configure their account in the Outlook App (version 4.57.0) and later whilst running iOS 14.0.1. The issue remains under investigation with Microsoft. In the mean-time it is recommended that users configure their account in the native mail app on their device.
UPDATE: 20/10/2020 – Microsoft have released a new version of the Outlook App (4.60.0). This has resolved the issue. On-Premise users wanting to access email via the Outlook App on iOS 14.0.1 are recommended to install version 4.60.0