Conversion factor cause error at the time of delivery note

I have set conversion factor as 1 nos=0.018 Square feet. Finish Good item has serial number.
it works perfect in quotation and sales order.
I am trying to create delivery note with following items:

I just simply added one custom field for conversion value because item specifications can change.

it shows error like this:

It says you don’t have enough serial numbers selected…
Are you sure you have 100 FG units with SL.No?