If a function returns a value, you can't simply call it. (which is a silly restriction in my opinion, but what can you do)
To call this function you need to use it in an expression. For example:
Set var to Functions.myFunctionName
Or let's say you have a function which returns an amount of health for any enemy, you can use in a condition:
System Compare Two Values: Functions.getHealth(Enemy.UID)>0