I know that this is an old subject BUT here is the easy solution if the flash does not fire on the D500.
If your flash mode is set correctly then when the flash is attached and turned on an orange bolt is shown on the far bottom right in the viewer ... if nothing appears there (or on the left of the Info screen you have a bolt with a cross over it ) then you need to set the Flash Mode to have the camera talk to the flash
There is no Flash Mode menu selection in the D500 menu (unlike the Z cameras) .. On the D500 you have to get to Flash Mode selection via buttons on the back of the camera.
Here is how to get to and change the Flash Mode
1) attach the flash and turn it and the camera on
2) On the back of the camera on the RHS .. press the white and black lettered Info buttion (not the i button) and the info screen will appear ... you can adjust shuter and fstop here.
3)now press the info button again .... and it will take you to the flash info screen
4) On the back of the camera on the LHS you will see the zoom out button ... and there is a little lightening bolt beside it. Press the zoom out button and you will then enter the Flash Mode menu
5) use the rear wheel (the one you would normally use for shutter speed) to choose the flash mode in the lower left had coner... HINT ... just use the Bolt alone with the TTL mode.. dont worry about red eye or rear screen unless you understand the implications on shutter speed etc.
6) the front wheel adjusts the flash compensation in this menu ... read up on this before you use it though.
7) Push the I button to set the mode.
If you have done it right then the orange bolt will be on the viewing screen
I like TTL-BL-FP selected on the flash and camera