In order to use Remote Desktop you have to have the MS Authenticator App as your default MFA method. The instructions below show you how to add the app as a method (if you don't already have it) & how to set it as default,
Add Authenticator App
1. Download the Microsoft Authenticator app onto your mobile (from the relevant app store for your device)
2. On your laptop/PC login to Outlook Web
3. Click on your face/name in the top right
4. Select View Account
5. Select Security Info (if you then see a blank screen, please try using a different browser)
6. Select Add Method
7. Select Authenticator app
and then follow the on-screen prompts
Change Authentication Default
Once you have the authenticator app in Security Info (as above) click Change next to Default Sign In Method and select Microsoft Authenticator - Notification from the drop down menu
More detailed instructions can be found in Multi-Factor Authentication (MFA) - Setup Instructions and step by step videos can be found here: https://servicedesk.runshaw.ac.uk:8081/servicedesk/customer/kb/view/71139476