Order & has already been combined

What causes this issue?

You tried to add a further original order to a combined order.
However, order &V1&, which you selected, is itself a combined order,
and therefore cannot be added to the combined order.

System Response

The system terminates order combination.

How to fix this error?

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.