When my 2000 Ford Explorer is parked on a incline it will not start until I park it on a level surface what could cause this.

First answer is extremely possible because your fuel pump is on the right rear of the truck so if you park uphill on the left side of the street/drivers side the fuel in the tank is leaning to the left and the rear of the tank and if there is almost now fuel in the tank (less then a 1/8-1/4 and that uphill incline is steep and to the left the fuel is not near the fuel pump inlet screen and will not start also the fuel pressure sensor in the tank top right side can be failing and is weakened from age your truck is 19-20 years old by date on your sticker on the door/jam so this could be 1 of 3 problems #3 is the fuel pump relay can also be so old that it to has weakened and only can function when level it also may be loose fitting in the box it plugs into so will loose fuses and the fuel pump connector and also the inertia switch that shuts of the fuel pump if an accident occurs but I really think it's your fuel level on a steep hill that's doing this you know what side of the street your parking on when it happens even downhill but on the left will keep low fuel from getting to the inlet those are the possible causes

The same thing happened to me many years ago. It was not out of gas but it was so low that the gas wasn't reaching the fuel pick up hose in the tank, at that particular angle.
I coasted down the hill to level ground and it started easily.
Try putting some gas in the car and try starting again on the hill. If it still won't start then something else is wrong, but I have have no idea what it could be.
Good luck