Modeling a company's ability to service debt

I'm working on a deal for a company that's looking to raise debt capital. I've built out the 3 statement model and incorporated base case debt assumptions but am looking to revise it for a variety of scenarios. It is my understanding that negative LFCF means that the business isn't able to service the debt but is this correct? Or at least, a lender wouldn't consider them eligible for the debt at those terms if LFCF is negative?