Why isn't my external flash going off?

I'm using a TT500 speedlite external flash and a Nikon D500 DSLR camera.i've got a radio slip set up so that the flash should go off when I hit the shutter. I've checked the channel the radio slip is on and tried turning it off and on. The batteries are full. Any help?

Added (1). Yeah, I did mean D5000

This flash is not a dedicated flash for use on your camera. (probably a D5000, not D500)

What radio control system are you using?

You are going to have to contact the maker of the flash unit and/or radio remote to figure this out.

Had you purchased a Nikon SB700, you could use your Nikon cameras CLS system on your camera

http://support.nikonusa.com/...stem-(cls)

Hopefully the makers of the TT550 and your radio remote will have such support

Make sure that you're shooting faster than the sync speed of your camera (for your camera 1/200 second), otherwise it might not trigger flash sync. Try taking the transmitter off your camera and shorting the contacts on the bottom (connecting them with a piece of metal such as aluminium foil), which should also trigger the remote flash.

To add to fhotace, check to c if your light on trigger is firing to confirm signal message is going threw.
Also, did this ever work before?
explain radio slip also-- what do you mean?