Value of cost component split is less than zero

What causes this issue?

The <DS:GLOS.cost_of_goods_manufactured>cost of goods manufactured</>
of the main product was more than compensated for by a by-product
because of a combination in the <DS:GLOS.quantity_structure>quantity
structure</> or because of an unrealistically high valuation price.
Because negative <DS:GLOS.cost_component_split>cost component splits</>
are not allowed in the system, the cost estimate is not saved.

How to fix this error?

Check the <DS:GLOS.bill_of_material>BOM</> for this material.
Check the valuation price of the by-product.

Error message extract from SAP system. Copyright SAP SE.