i have an asus i7 windows 8.1 system but i cannot run any 3d graphic game. why?

It seems like your PC is an OEM prebuild. Check with ASUS on this because you will likely need to upgrade the power supply unit to install a GPU and that may void the PC's warranty. If ASUS will let you upgrade without voiding the warranty, then get yourself an XFX 550w power supply and a GTX 960 or above.