I also had the problem that Outlook could not be switched to the new mode. I solved this by manually adjusting the settings with the terminal:
Attention: the settings are case-sensitive!
1) First, let's see if there is a corresponding value:
defaults read com.microsoft.Outlook EnableNewOutlook
Result:
2021-03-23 12:17:53.102 defaults[45426:841968]
The domain/default pair of (/Users/Username/Library/Containers/com.microsoft.Outlook/Data/Library/Preferences/com.microsoft.Outlook, EnableNewOutlook) does not exist
2) Writing the settings:
defaults write com.microsoft.Outlook EnableNewOutlook -integer 3
3) Checking the settings:
defaults read com.microsoft.Outlook EnableNewOutlook
Result:
3
Possible options:
0 = Switch hidden (default)
1 = Switch displayed, default off
2 = Switch displayed, default on
3 = New Outlook enabled with switch hidden
With "defaults read" and "defaults write" values in the corresponding settings files (PList) are adjusted.
NOTE: Please use this way only if you know what you are doing 🙂