The reference document has not been completely invoiced.

What causes this issue?

You attempted to create multiple invoices for one reference document in
conjunction with split invoicing. You already created a partial
invoice, but you have not created invoices for all invoicing cycles. If
you were to create this document, you would not be able to create
billing documents for the un-invoiced cycles.

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?

Bill the un-invoiced cycles first, before you post new quantities that
have to be invoiced.

Error message extract from SAP system. Copyright SAP SE.