PO quantity smaller than requisition quantity

What causes this issue?

This purchase requisition has been created from a
<GL:sales_and_distribution_document>sales document</>. It can only be
copied into a single purchase order item, that is, it is not possible
to reference the requisition in more than one order item. If you want
to order only part of the total quantity, the remaining quantity of
this requisition will remain open.
You will then only be able to close this requisition by changing the
relevant sales document.
Neither is it possible for the sales document itself to be completely
closed as a result of a partial order.

How to fix this error?

Since a sales document containing this quantity exists in the system
for this transaction, you should copy the entire requisition quantity
into the PO item.

Error message extract from SAP system. Copyright SAP SE.