Purchase order & does not exist

What causes this issue?

You have entered a purchase order number that does not exist in the
system.

How to fix this error?

Press ENTER and correct your entry.
If the purchase order number has been wrongly quoted in your source
document, you can search for the correct number by positioning the
cursor on the field <LS>Purchase order</> and pressing F4. You can then
display all the purchase orders relating to the supplier or to the
materials listed in the invoice.

Error message extract from SAP system. Copyright SAP SE.