Order type & has internal number assignment (entry not required)

What causes this issue?

You have entered an order number, ben the the order type of the order
specifies internal number assignment (i.e. assignment by the system).

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?

Do not enter a number. The sytem will automatically assign a number to
the order.

Error message extract from SAP system. Copyright SAP SE.