The problem is
data = windTurbineModel(windTimeSeries,Cd(j),deltaT,time,k(i));
The function windTurbineModel does not return any data, so it throws an error. Exactlye what variable inside the function were you expecting to return? You need to add it to the function definition line, like
function omegaPower = windTurbineModel(Windspeeds,Cd,deltaT,time,k)
