PAR.Ac = (pi/4)*((PAR.D1t^2)-(PAR.D1h^2)); %Compressor area
There's no information for your PAR.D1t and PAR.D1h. I guess these are diameters. Consider adding these:
PAR.D1t = input ('Insert outer diameter (m) ' )
PAR.D1h = input ('Insert inner diameter (m) ')