It is better to put the actual code in your question rather than just screenshots. But from your picture it looks as though you are expecting a single value from your sum() function? Try putting a breakpoint on line 37, run your code, then manually type (copy) sum(y(k:k+mn-1).*PN) into the command window to see the result. If the result is not what you expected, then there's your issue.
I've no idea what you're trying to do, so that's the best I can give you.
