I am using Odoo v8 with Centos 7

Everything works fine

I have a problem inserting the components of the BOM

I have components that I have to put their positive quantity and components with a negative quantity

I have a duress error message:

All quantities must be greater than 0

You must Install the mrp_byproduct module if you want to manage additional products in the BOMs!

I installed the mrp_byproduct module and it didn't give anything

Thank you in advance for your assistance