Reference material not yet maintained for this transaction/event

What causes this issue?

You have chosen a reference material whose data has not yet been
created for this transaction/event.

How to fix this error?

INCLUDE NA_EINGABE_PRUEFEN
INCLUDE NA_EINGABE_FALSCH
If your entry is correct, you must create the master record accordingly
before you can use the material as a reference.

Error message extract from SAP system. Copyright SAP SE.