LVL UP TEXT EVENT

At beginning of game:
set a variable equal to your character's starting level

Parallel process:
set a second variable equal to the character's level
conditional branch: if the second variable is greater than the first variable...
> then show the text "LEVEL UP"
> and set the first variable equal to the character's level
end condition

repeat all of the above for every character. use different variables for every character.

I recommend putting the "LEVEL UP" text in a battle animation instead of actually using the Show Text event command, so the player won't have to hit enter.

Yeah, in this case, the engine didn't matter because this is literally the simplest thing of all time. When the first cavemen were trying to figure out how to make fire, millions of years ago, they gave up after about an hour, and decided to make a level up event instead because it was simpler.

But in general please always tell us what program you're using.