Ribas and Hill were scheduled to fight on the main card, which is headlined by Marina Rodriguez and Michelle Waterson's women's flyweight clash.
The UFC removed two fights from the event Friday. Ryan Benoit was three pounds overweight ahead of his fight with Zarrukh Adashev, while Philipe Lins didn't receive the necessary medical clearance to take on Ben Rothwell.
Ribas and Hill are the No. 11- and 12-ranked strawweight contenders, respectively.
Ribas is 10-2, most recently losing to Rodriguez by second-round TKO at UFC 257 in January. Before that, she had been riding a five-fight winning streak, with four of those victories coming after she joined UFC.
Hill rebounded from losses to Claudia Gadelha and Waterson to earn a unanimous decision over Ashley Yoder at UFC Fight Night 187 in March.
Particularly for the 27-year-old Ribas, UFC Vegas 26 presented an opportunity to continue climbing up the division.