You don't provide a lot to go on, so I'm inferring that you are new to this functionality. Therefore, let's get real basic.
1. When you set up your data, did you happen to also set up an on-hand inventory balance for the Co-product? If so, and it was adequate to cover the demand, material planning would see no need for a work order.
2. Check the processing options and the data selection criteria of the MPS version you ran. (You did mean MPS and not MRP, correct?). Are those processing options and selection criteria consistent with the planning parameters set in the Item Branch record (like, for example, the Plant Manufacturing tab on the Additional System Info row exit)?
3. Is the Doc type / line type combo for the sales order you are using defined in the Inclusion Rule used by your MPS version? Is the shop floor calendar set up for the B/P over the planning horizon of the MPS generation?
4. Are sales and manufacturing for the Co-product set up in the same B/P? If not, did you set up B/P relationships and execute a multifacility version of MPS?
5. If all the above are "correct", MPS still will not produce a work order - it will recognize the demand and set an 'O" (order) message for the Mfg process. When you process the message, it will create a work order to run the process.
6. If you happen to have set up a rate schedule, and it is adequate to cover the demand, the system presumes adequate supply. Otherwise it would set an "increase rate" message (and you still would not have a work order).
If none of the above help, we can take this off line. e-mail me directly.