I have two folders and cannot delete them, neither online nor in the windows app (v11.39). After deletion, a message is shown “Folder was deleted successfully” and then after refreshing the page the folders appear again.
Please check your logs what is going on, the links to the folders are:
https://web.postman.co/workspace/My-Workspace~adf36924-43b0-4b49-8722-d9bf61242528/folder/41363365-75e06d86-57c5-494a-b3bf-f3d616173a48
https://web.postman.co/workspace/My-Workspace~adf36924-43b0-4b49-8722-d9bf61242528/folder/41363365-b00e379c-a2bf-416f-b991-c260f7ffc982
Welcome to the Postman Community! 
I’m not able to reproduce that on my side - Are these old folders or new folders that have just been created?
If you create another folder, can that be deleted?
Have you tried closing and reopening the client or logging out and in again?
On the desktop client, have you tried the Clear Cache and Reload option?
If you’re still having issues with this, could you share a screen recording showing the problem on your side?