mkdir cannot create a particular directory name
CON is a reserved name (originated with MS-DOS) it was short for console which was the original text based output screen. Others are PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9