You cannot change the quantity of the product receipt (manuf.co-products)

What causes this issue?

You want to change the quantity of a planned order. The planned order
has several product receipts. This is manufacture of co-products. In
this case, you can only change the quantity of the planned order using
the product for which the planned order was originally created. You are
trying to change the quantity of the planned order using a different
product.

System Response

The planned order quantity is not changed.

How to fix this error?

Change the planned order quantity for the product for which the planned
order was originally created.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.