Error message: unable to complete authentication flow

Hi Siembida, 

Sorry to hear that you’re running into the “unable to complete authentication flow” error in Outlook for iOS.  

Before we proceed, please note that forum moderators do not have access to user account settings and cannot assist with logging in, resetting passwords, or changing access rights. While we do not have access to internal systems or administrative tools required to resolve account-specific or backend-related issues but we’ll continue doing our best to support you within the scope of our responsibilities.    

This type of error is commonly related to the authentication step not completing properly between Outlook and the authentication app on your device, such as Microsoft Authenticator, Duo Mobile, or another MFA app. In many cases, the password may be accepted, but the final authentication response does not successfully return to Outlook. 

Here are some steps you can try: 

1. Fully close Outlook and any authentication apps on your iPhone, such as Microsoft Authenticator or Duo Mobile, then reopen Outlook and try signing in again. 
2. When you receive the authentication or MFA prompt, approve the request and allow the authentication app to finish the process. If the sign-in flow redirects you between apps, please avoid switching back to Outlook too quickly. 
3. Make sure Outlook and your authentication app are updated to the latest version from the App Store. 
4. Confirm that notifications are enabled for the authentication app, as the sign-in process may fail if the MFA prompt cannot be delivered or approved correctly. 
5. If you are connected to VPN, please temporarily turn it off and try signing in again. You can also try switching between Wi-Fi and mobile data to rule out a network-related issue. 
6. As a final step, remove the account from the Outlook app and add it back again to start a fresh sign-in session. 

After completing these steps, please try signing in to Outlook again. 

For you reference: Sign in using Microsoft Authenticator | Microsoft Support 

If the issue still persists, please reach out to your school or university IT admin for further assistance, as they may need to review or reset your MFA/security settings. You can also share the steps outlined in the article Manage user authentication methods for Microsoft Entra multifactor authentication. This will help them assist you in resetting your MFA settings and restoring access to your account.  

• In case you do not know who is your IT admin, kindly refer to this article: How do I find my Microsoft 365 admin? - Microsoft Support  

I hope this can be resolved soon. Should you have any further questions or need additional assistance, please feel free to share them in the comment below. I'm very happy to help.  

Thank you again for your patience and understanding. 

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