My gas lawn mower keeps spitting off the spark plug. Just replaced spark plug recently. What could be the problem?

tighten the spark plug.

The threaded hole in the (probably aluminum) cylinder head must be damaged.

If you don't know how to repair it, just get another lawnmower. It costs too much to pay someone else to do it.

Either the new spark plug is TOO SMALL, or you destroyed the threads.

what does "spitting off" mean?

the spark plug is the WRONG size === do not operate the mower until it is corrected ____ in the present condition you can ruin the motor ............

Not only could the plug be cross threaded but you may also have not tightened the plug enough. Finally you might have the wrong plug size.

You may have cross threaded and destroyed the threads. If so, you need a helicoil. It is like a spring that replaces a worn thread. You might have to take it to a service repair place or talk to your local hardware store people about it.