There has to be a foot valve on the end of the pipe or both pipes going down the well. If the foot valve leaks, you will not be able to prime the pump. You fill the pipe or pipes going down the well as you pour water into the top pf the pump, usually through a plug you remove from the outlet pipe from the pump. If there is a leak between the pump and the foot valve, you will have problems priming and if you do get it prime, then, after priming, the pump will come on (and off if the pum pcan build enough pressure) when you aren't using water and eventually that can ruin a pump. Or if the leak is large enough, the pump won't be able to build enoug hpressure to shut off or, you'll get air in the water.
If there is a check valve above ground, and a bad foot valve on the end of the pipe or pipes in the well, the check valve can hide a leak and it will prevent you from priming the line past the check valve and down the well.