Push it in very good you shouldn’t be able to just take it out it should have a lot of resistance when pulled on. I had to use a pair of pliers before (not recommended) but just to give you an idea. Just try to push it as hard as you can.
Also do yourself a favor dump the spark plug in the garbage and get yourself NGK spark plug the size is CM6 you could order it online or you might be able to find it at your local auto parts store.