wow.. your battery is dead.. charge it or replace it. A battery pack jump starter's wires are too thin to carry the current to turn a starter when the battery is completely dead.
I've seen it a hundred times, person swears the battery is good, people tell him to pull plugs, check valves.. just put a new battery on it, if you do not want to spend 50% on a new battery to test it out, get yourself a good quality 2awg or 0awg jumper cables, hook it to the car with the car running, attach it to the tractor, let it try to charge the tractors battery for at least 15 minutes and then try to crank it over..
Portable jumpstarters are not for completly dead batteries, unless you got one of the 900amp cranking power ones for like 300$+. Also, if the battery in the tractor did in fact go 100% dead, chances are there is now a short between the internal cells, this alone can draw up to 250 amps from your jump pack. those batteries are not designed to go below 70% charge at 0% few if any can be recovered. Personally I would just replace the battery, they are less then 50$ at walmart.