In call to function My_sum, the first parameter, X, should be in descending order.
You have called a function that requires the values in the array passed to the parameter to be in descending order, but the values in the array were not in descending order.
For a function parameter declared as follows,
Function My_sum(X : descending))
calling the function as called below would trigger this error as the array passed is not in descending order.
My_sum([4, 1, 7])
Pass an array with values in descending order as the indicated parameter to the function.
You can use the Sort function to sort an array in ascending or descending order.