The error message is indicating that either you are passing an incorrect set of input parameters to the portalpha function, or you don't have this function (which is part of the Financial Toolbox). In the Command Window type
which portalpha -all
to list the path to the function. If you see something like 'portalpha' not found., then you do not have it (or at least, it is nowhere along the MATLAB search path). Also, check to see if you have the Financial Toolbox by typing
ver
to see the version of MATLAB and all installed toolboxes.
