The system does not define the technical type automatically. First you must create an additional alternative for or a variant of an existing simple material BOM.
If you define a BOM as technical type "multiple BOM" when you first create a simple material BOM, you can maintain the lot-size fields.
Please note that you cannot change the technical type once you have set it.
How to define the technical type of a BOM:
You see the header screen from which you called the function again.
If you selected technical type "multiple BOM", you see the fields for lot size on the Quantities/long text header detail screen.