PO not selectable (curr. & /exch. rate & fixed)

What causes this issue?

A fixed currency exchange rate has been specified in the purchase order.
The document you are currently processing specifies a different
exchange rate or a different currency. Because of this you cannot select
this purchase order.

How to fix this error?

You must terminate the selection and process the invoice item for this
purchase order separately, in a document of its own.

Error message extract from SAP system. Copyright SAP SE.