'item' is a lookup field and child fields cannot be accessed

'item' is a lookup field and child fields cannot be accessed

Hi,  I have 2 forms 

First  with product name and rate
second quantity and amount

i want to multiply rate from parent form field  with quantity of child form field and display in amount field. Please help new to zoho creator.. I get  "'item' is a lookup field and child fields cannot be accessed" error

thanks
anand