Hi Rahul,
After looking at the code, I think that's a "bug", and you should open an Incident on OSS for a fix.
The "supplementary quantity" and associated unit fields are only open when the Tariff Code needs such information. And in most of the incompletion check modules, the developers have coded accordingly.
But they've forgotten in this case. You might want to compare the US code in Form FTZ_QUANTITY_CHK (program /SAPSLL/LINCOMP_CHECK_USF05) with the UK code in Form QUA012_GB_CHK (program /SAPSLL/LINCOMP_CHECK_GBF01). It's easy to see that there's a missing line in the US code: IF ( sy-subrc EQ 0 ). That means the quantity and unit check is always carried out, even if there is no need.
I hope you get it fixed ok.
Regards,
Dave