@Shumbody, welcome to the forum.
Are we talking about this nut (pls see pic)?
-If is this the case, it might be necessary to remove the wheel and check whether one or both threads have suffered.
-only putting back the axle nut without being sure its unharmed or not, I consider too risky
-the nut is not a very hard metal
-axle material in comparison is hardened/nitrated
-so it's more likely that, your nut has lost its thread, means you need a new nut.
This nut is not to get at hardware store, but ordered from a Kymco dealer of your choice.
Check all this out and ask here again, if something seems to be unclear.