Remove the shutter cable. That is for using another RF-603II to remotely trigger the camera's shutter. This seems to be causing your trigger to activate the flash as soon as the camera's shutter button is pressed but before the first shutter curtain is open.

Your camera communicates the order to fire the flash via the main contact on the hot shoe. With a manual only trigger such as the RF-603II you need to be sure your camera's flash mode is set to manual flash. E-TTL mode might cause the pre-flash signal to set off your flash.

Since your flash is set to manual power, the "pre-flash" (whether caused by the shutter cable or an E-TTL setting) would likely be at high enough power to drain the capacitor in the flash enough that there would not be enough energy in the capacitor to fire again so quickly when the "fire" command is sent from the camera once the first curtain has completely opened.

Other things to consider: Be sure the flash and trigger have fresh batteries. Be sure the trigger on the camera is set to "TX" and not "RTX". Be sure the trigger is seated properly on the hot shoe (It's common for some folks to push them on backwards). Be sure the receiver built in to the flash is set to "RX" and not one of the optical slave modes (S1/S2/Sc/Sn). Be sure any silent shooting options your camera may have are disabled, as they will sometimes disable the flash from firing.