Order &1: Main material does not exist

What causes this issue?

The main product does not exist in APO. Either you have not added this
product to the integration model or this product is not to be planned
in APO and has been transferred due to entries made for the production
orders.

System Response

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

How to fix this error?

Check the integration model in the R/3 System:
If the product is APO-relevant, add the product to the integration
model.
If the product is not APO-relevant, check the production orders in the
integration model and delete any from the model for this product.

Error message extract from SAP system. Copyright SAP SE.