Matlab : How to plot the constellation diagram for this method?
The function you are using is from the Communication Systems Design Toolbox. You might have it not installed/licensed on your PC.

In case you just want to plot a constellation diagram, you can go with this code (Note, that it works for QAM constellations, not BPSK (since BPSK has only one domain)).

values = -7:2:7;  % Values in real and complex domain
[X, Y] = meshgrid(values, values);
QAM = X(:) + 1j*Y(:);
plot(QAM, 'x')


Actually, the QAM constellation contains all combinations of possible real and complex values. Hence, you need the cartesian product between real and complex parts. Taking this solution, you can plot the constellation diagram.