Adding Quantity field in Appended struncture

Hi,

I am new in ABAP and the scenario is i have a structure appended in table and I want to add the quantity filed VOLUM in that structure it gives me the error although i have mentioned the field name with the table name i.e. VBAP-VOLUM but it says type is ont active.

kindly help me in this regard.

Thanks

Mohammad Jawad