Right-click the error and select "Trace Error".
Then select "First".
It takes you to the for loop in the "Iter_neu" function. You have specified the end of your for loop to be last(G), but you have not defined G. Therefore, Mathcad sees G as the built-in dimension for magnetic-flux density, Gauss, which is not a vector so Mathcad cannot accept it in the function last(G).
After looking at your program a little more, I see that you are calling Inter_neu within Inter_neu. Is that intentional? What are you trying to achieve?
One more observation for you: your for loop can only contain 2,147,483,647 elements (2^31 - 1). That's the largest number for a 32-bit integer. So in your first "Iter" function, the for loop cannot go from 1 to 10^99. If you try, it will give the error "Object reference not set to an instance of an object."
I took a guess at what you were trying to do and attached it here.