If Score % 20000 = 0
What if you are at 19999 points and gain 10 points? It skips over 20000, which means the above condition is only run if you hit exactly 20000, 40000 or 60000 score.
If the score doesn't decrease, I'd suggest something like this:
+ if Score/20000 > var
Lives = Lives +1
Var = Var +1
What does this do? Well, it tracks how many times you gained life by scores! When you hit 20000 score for the first time, the above condition is true since var = 0 at the start and Score/20000 = 1 > 0. Once the event is processed, it also increases var by 1, so that it won't run again until you pass the next milestone.
Meanwhile, you can abuse lives all you want!