Brett Okamoto of ESPN also said multiple sources told him the fight is "100 percent off."
Ariel Helwani of MMA Fighting first reported the holdup, noting the 29-year-old was feeling unwell all week.
Nunes was set to defend her women's bantamweight title against Valentina Shevchenko. The two first faced each other at UFC 196 in March 2016, and Nunes won by unanimous decision.
As Mike Bohn of Rolling Stone pointed out, Saturday's highly anticipated matchup was set to introduce a "new era in the UFC women's bantamweight division," as it is the first title bout that doesn't feature a star such as Ronda Rousey, Miesha Tate or Holly Holm.
Nunes defeated Tate at UFC 200 last July to claim the title before defending it against Rousey at UFC 207. Despite the loss, Shevchenko provided tough competition the last time she faced off against Nunes—she outlanded her 17-3 in their final round, according to FightMetric.com—and she earned an impressive win over Holm several months later.
Bleacher Report's Scott Harris speculated on a possible cause of Nunes' illness:
Yoel Romero and Robert Whittaker will battle for the UFC interim middleweight championship Saturday in what was supposed to be a co-headliner.