I am designing a game where if you press space it opens or closes a menu, but I am unclear of how to do that. Do you know how to?

You could set a variable with an on/off or 1/0 setting to represent whether it is open or closed.

when green flag clicked
set [Menu v] to [0]

when [space v] key pressed
if <[Menu] = [0]> then 
set [Menu v] to [1]
show
  

else
set [Menu v] to [0]
hide

end

I think this would work.