Suddenly I cannot create a new folder in WordPerfect X9 (Windows 10 Pro). I can save a file to a previously created folder, but cannot create a new folder.
How are you trying to create an new folder? Can you create a new folder using the file explorer (whatever it's called - maybe just Explorer) in Windows 10?
I have been trying to create a folder within WordPerfect. I find I cannot create a new folder in file explorer either, which I believe means it's not a WordPerfect issue but rather a Windows issue.
If not already, try logging on as an Administrator.
And/or check your User Account Control settings.