Do not fill the "Document Number" field

What causes this issue?

You have entered a document type that requires an internal number
assignment. In this case, the <LS>Document Number</> field must remain
empty.

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?

Please delete the number in the <LS>Document Number</> field.
If you want to specify an external document number in the document, you
can use the <LS>Reference</> field.

Error message extract from SAP system. Copyright SAP SE.