Please don't YELL!
Next, NEVER define pi. pi already exists, and I hate to tell you, it ain't 3.14. That only happens in certain places I won't name. In MATLAB, pi is already defined, far more accurately than that.
ySol(x) = dsolve(ode,y(0) == 0,y(pi) == 0)
Which is what you would expect, since if we have the ODE with no boundary conditions at all,
y'' == 0
the general solution is as you found, a linear polynomial. If that polynomial must then be zero at two points, then the polynomial reduces to the constant value 0.