There’s a syntax error in your calculated field. One of your closing brackets is in the wrong position. The truncDate function doesn’t take ASC as argument. ASC should be part of the runningSum function and not in truncDate.
To see the syntax error more clearly, you can write the calculated fields separately instead of nesting them.
Month = truncDate(“MM”, {created_at})
Running Sum = runningSum( sum({Contract Id}), [{Month} ASC] )