Problem to program ATxmega256 whith command line

HI,

I want to program my µc ATxmega256A3 whith line command using STK600, so 
i write "STK600.EXE -dATxmega256A3 -cUSB -e -ifRC1.hex -pf -vf"
But i have always an error : "Could not connect to STK600 on USB".
Idon' know what's the problem.
If you have an idea please to answer me .

Thanks