can't delete folders via FTP (FileZilla)

Greetings!
I used FileZilla to create three folders in Quixpin, and placed music for 3 different events in each folder.
No problem accessing these files in Quixpin, and no problem deleting the files via FileZilla after the events.
However, I cannot seem to delete the folders themselves.  FileZilla reports the following error:
Command: RMD The Play
Response: 500 'RMD': command not understood.
How do I delete folders from Quixpin?
Thanks,
Garrett