Epps, who would have been the first African-American crew member on board the ISS, was pulled from her mission for unspecified reasons.
"A number of factors are considered when making flight assignments. However, these decisions are personnel matters for which NASA doesn't provide information," Brandi Dean, spokesperson at the Johnson Space Center was quoted as telling Arstechnica.
Epps was a member of NASA's 20th class of astronauts, a group of nine known as the "Chumps" who were selected in June 2009.
Only three other African American women have flown into space. Epps' assignment in January 2017 garnered a fair amount of favourable publicity for the space agency.
She had been assigned to her first spaceflight, Expedition 56/57, scheduled to launch in May 2018.
Seven of the nine astronauts from that class have already flown into space.
Epps, a native from New York, will be replaced by the other novice from the 2009 class, Serena Aunon-Chancellor, who was serving as Epps' backup for this mission.
That was because the primary crew was exposed to rubella -- a contagious viral infection -- and Mattingly was not immune from the disease.
NASA does not usually say why crews are reassigned unless there is a medical reason, the report said.