the "debt" in FCFF calculation is actually total liability. In financial literature, debt and liability they are sometimes used interchangeably, and that is why confusion arises.
In a precise sense: Liability = operational liability( ex. A/P, deferred tax liability) + debt( notes payable, current portion of LT debt, LT debt)
regarding EV calculation. The enterprise value takes a market perspective, i.e. it only takes into account the claims that can be traded in the market. Notes payable is an debt contract that cannot be traded in the "market", unlike publicly traded fixed income instruments. So there is no market value for notes payable.