Not sure how to blit font with a variable onto a surface, pygame

Question

trying to blit onto the screen a font with a variable - the score you have achieved - onto the screen after dying.

BLACK = (0, 0, 0)
font_small = pygame.font.SysFont("Verdana", 20)
scoreMsg = "Your score: {0}".format(SCORE)
show_score = font_small.render(scoreMsg, True, BLACK)

later on I call the show_score variable like this:

screen.blit(show_score, (30, 400))

here is the full code: https://pastebin.com/5RnShSCG

edit: i forgot to mention. the text "Your score:" shows up on the screen, but the variable is always 0, even if score is higher.