Material assignment to sub-operations not permitted

What causes this issue?

You selected the material component overview for a sub-operation.

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

System Response

The system does not let you allocate material components to sub-
operations; it cannot display the overview for a sub-operation.

How to fix this error?

You can only allocate the material component to the operation to which
the sub-operation belongs.

Error message extract from SAP system. Copyright SAP SE.