I am confused!
You seem to know the solution, so what is the purposeof the question?
Just to see if Mathcad is capable of doing it? You saw that it is not and you have been told over and over again that Mathcad is not the right tool for that kind of calculations which you are trying to do all the time (whatever that may be anyway).
So again - Mathcad is not the right tool! It gets even worse if you partially simplify it manually and replace the inner sum by its equivalent closed expression and then tell Mathcad's symbolic to evaluate it. So if you want some fun, her you are:
Simply replacing the expression (-1)^j by the equivalent cos(pi*j) gives you the result you expected
And whats the benefit? To what extent will this help you now?