My friend's diesel tractor can't ignite, nor can it ignite with a crank. What is the reason?

This question is too big to say clearly at once. Diesel engines ignite fuel by compression. First, they need enough compression. So first check whether the cylinder leaks: crank the crankshaft by hand, don't decompress, and see if you can shake it once. If you can't prove that the cylinder has no oil leakage, and if the resistance is small when shaking, you can turn it to prove that the cylinder has oil leakage, and you need to replace piston rings and other parts. This is the basis for the normal operation of diesel engines. Secondly, accurate oil supply time and good spray quality are required. This is something that non-professionals can do well. You repair the cylinder and the repairman will solve it together.