What is a 1095-S? I have never seen one.....

Meant 1095-A

Column C can be blank, but YOU need to find the amount for Column B using their SLCSP lookup tool

https://www.healthcare.gov/tax-tool/

if column c is blank then they did not get any help or subsidy so if you just put $1 in column b you can get past the error - column b is needed to calculate with column c for if they owe money back or if the gov't can give them more help

They may be entitled to some PTC, so you need to find the right amounts for Column B or you could be leaving money on the table.