Predecessor cannot be found: Source Transaction AP_VOUCHER, Referenced Source Transaction PO_POENC

Does anyone know what this means.  It is a budget checking error on a voucher.  Predecessor cannot be found: Source Transaction AP_VOUCHER, Referenced Source Transaction PO_POENC. This line references a predecessor, but the predecessor cannot be found.  Either the predecessor is missing, the predecessor failed budget checking, the Referenced Record Keys are not defined correctly on source transaction mentioned above, or the Header/Line Records are not defined on referenced source transaction mentioned above.  Please check all the possibilities as stated.

Thank You