NASA rescheduled its Artemis I launch after a 2nd attempt to get the mission off the ground.
The space agency is preparing to launch its Space Launch System rocket and Orion capsule.
On Saturday morning, as NASA was filling the rocket, the agency's team discovered a hydrogen fuel leak from the engine section.
NASA made several attempts to repair the leak, but time ran out before the launch window opened at 2:17 p.m. ET.
NASA canceled a first launch attempt on Monday after failing to resolve a temperature issue with one of the rocket's four liquid-fueled engines.
It was unclear whether NASA would be able to try again on its next launch opportunities on Monday or Tuesday.
The unmanned launch will mark the debut of the most powerful rocket ever assembled, as well as the beginning of NASA's long-awaited return to the moon's surface.
It is the first mission in NASA's Artemis lunar program, which aims to land astronauts on the moon by the third mission in 2025.