Purchase requisition &, item &, does not exist

What causes this issue?

The number of the purchase requisition or item you entered does not
exist.

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?

INCLUDE NA_EINGABE_PRUEFEN
Choose the function "Possible entries" to find the number of the
desired purchase requisition.

Error message extract from SAP system. Copyright SAP SE.