the max() expression returns the smallest value of any amount of numbers given
so
max(4,2,6)
will return 2
in your case you can do:
set score to max(score,0)
which means if score is below 0, it will just return 0
min() works in the opposite way
other related expressions are clamp() and abs(), you can find them with a description in the 'objects with expressions' window, in system.