Beginner need help

 
Hallo, I'm a beginner in programming MWL4 and I've the following Problem: I want to calculate the difference between the High and Low of the last twenty bars. In the next step I want to search for the highest difference of the last twenty bars and show it like an indicator. Hier is my Code, but it must have some mistakes. Best regards, Jurdop
 
Try giving us the source and not a picture of the source.