This usually happens when the denominator is negative. For example, for a start-up tech or biotech company, earnings may be negative, so price / earnings would be negative. If EBITDA is negative, then EV / EBITDA would be negative too.
Instead of displaying the negative multiple, we replace it with the initials NMF, which stands for not meaningful.
For companies that are not making positive earnings or EBITDA, EV / revenue may be a more useful metric to gauge valuation. For example, venture capital investments are commonly judged on the basis of EV / Revenue.