Hi @shona schonning ,
Welcome to our forum!
Have you tried to select the folder and then click Folder via File>New or just right click the folder and click New Folder?
If the above doesn't help, are you using a Gmail account? If so, as i know, it's an expected behavior in Outlook for Mac. Please refer to my reply in this thread:
[Cannot create folders in Outlook on Mac (desktop version)](https://learn.microsoft.com/en-us/answers/questions/1193431/cannot-create-folders-in-outlook-on-mac-(desktop-v)
Hope the above help. If the above is not your case, please provide more information about your case.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.