Windows or MS-DOS don't let you to create these folders simply

There are some reserved keywords for windows to perform some specific tasks like write and store data, printing-related tasks, and other specific tasks. 

Those names are CON, PRN, AUX, NUL, A: - Z, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9 

As these keywords are reserved to perform some tasks by the OS windows doesn't let you create folders with these names. 

It uses these words to create folders from time to time and store data in them for tasks related to printing and other functions. Creating a folder with the same name confuses the system where to write data or create folders. 

Also, you can't add extensions with these names. 

The function of given keywords are given below : 

CON : Keyboard and Display 

PRN: System Device, a parallel port 

AUX : Auxiliary Device, a serial port 

NUL : Bit bucket device 

A: to Z : These are drive names 

COM1 to COM9 : Serial communication ports 

LPT1 to LPT9 : Parallel printer ports 

Though you can't create these folder names, you can use the command prompt, or blank space code to create folders with reserved names in Windows. 

Hope you have understood this. You can comment if you have any questions. Stay tuned for more information. Share with others to let them know. Thank you.