Suggestions:
Site code Exc is mandatory while making the new site in site master. Users have entered site code which is defined in site master in the field ‘Site code Exc’.
Site code Exc is basically a GST registered site, in one state even if more sites are there GST registration will be of one main site entered in Site code Exc.
When you put the new site code as in site code exc , As the site code exc is mandatory then it will not validate the site existence in site master. Sales dispatch validates the field on confirmation.