Looks to be a standard bung to me. You have two options, one buy the correct tool or do like i Do. Take two pry bars, one stands vertical the other is crossways to that bar an behind the lip of the other side. Twist the crossways bar, counter clockwise to loosen. I always do 55 gallon bungs like that, I'm too cheap to buy the proper tool, myself.
Once you know how a device, system, ect operates, finding the root cause in the event chain is easy. Until then, you will always be chasing/repairing the effects. Learning is a life time event, if you want to outsmart, whatever item you want to repair.