serial no 's of items should be displayed in the invoice

suri

hi good morning one and all
i have a question like this
serial no's of items should be displayed in the invoices
i have tried like this....
we have invoice form
card form=purchase header
subform=purchase line

along this above info i have tried like this as shown below
i have taken one data item
item
on after get record()

pline."No.":=Item."No.";
pline.MODIFY;
it was throwing an error
can any one suggest me is that i am doing correct or wrong
thank you

suri

i have also tried like this
in purchase line subform
no onvalidate()
if itm.FINDFIRST then
"No.":=itm."No.";
MODIFY;

but it was not working

suri

while i am writing the code in
form oninit()

IF itm.FINDFIRST THEN
"No.":=itm."No.";
MODIFY;

it was throwing an error
as shown below