Error faced when plotting velocity using stamps in Matlab

I am new to PSInsar. I am following various tutorials using stamps and matlab to measure deformation. I am using linux os. I was able to execute all the steps from 1 to 7 without problems of stamps manual guide. Then I faced a problem when i used the command
ps_plot('v")

The error is

Could not check if this is Envisat 
360482 ref PS selected
Interferogram (1) does not have a reference area
Array indices must be positive integers or logical values.

Error in ps_plot (line 1963)
        maxph=phsort(round(length(phsort)*.999));

why it is saying “Interferogram (1) does not have a reference area”. I followed the tutorial of RUS Copernicus in youtube. RUS Webinar Tutorial. I have also attached screenshort for reference.