NASA called off the test flight on Monday of its largest-ever Moon rocket because of a temperature issue with one of the four giant engines.
"The launch director has called a scrub for the day," the US space agency said.
Alternative dates for launch of the Artemis 1 mission, an uncrewed flight around the Moon as part of an ambitious program to eventually go to Mars, are Friday and next Monday.
Blastoff had been planned for 8:33 am (1233 GMT) but was put on hold because of a temperature problem with one of the four engines on the 322-foot (98-meter) Space Launch System (SLS) rocket.
Tens of thousands of people -- including US Vice President Kamala Harris -- are expected to gather along the beach to watch the launch, which has been decades in the making.
The goal of the flight, dubbed Artemis 1, is to test the SLS and the Orion crew capsule that sits atop the rocket.
Overnight operations to fill the rocket with more than three million liters of ultra-cold liquid hydrogen and oxygen were briefly delayed by a high risk of lightning, though it was a "go" after an hour.
Around 3am, another hiccup emerged: a potential leak was detected during the filling of the main stage with hydrogen, causing a pause. After tests, the flow resumed.
"The leak is at an acceptable level and we have returned to fast fill operations," NASA's Exploration Ground Systems tweeted, adding that they would continue monitoring.
While liftoff is scheduled for 8:33am, there is a two-hour window during which NASA has said there is an 80 percent chance of acceptable weather.
After the slight delays during fueling operations, NASA said it will determine a new launch time within that window.
The massive orange-and-white rocket, which has been sitting on the space center's Launch Complex 39B for more than a week, will not be able to take off in case of rains and storms.
The rocket's Orion capsule is set to orbit the Moon to see if the vessel is safe for people in the near future. At some point, Artemis aims to put a woman and a person of color on the Moon for the first time.
"This mission goes with a lot of hopes and dreams of a lot of people. And we now are the Artemis generation," NASA Administrator Bill Nelson said Saturday.
In another first, a woman -- Charlie Blackwell-Thompson -- will give the final green light for liftoff.
Women now account for 30 percent of the control room staff, compared to one for the Apollo 11 mission -- the first time astronauts landed on the moon in 1969.
During the 42-day trip, the Orion capsule will orbit the Moon, coming within 60 miles (100 kilometers) at its closest approach, and then fire its engines to shoot out 40,000 miles -- a record for a spacecraft rated to carry humans.
Besides the weather, any technical snafu could delay liftoff at the last minute, NASA officials have said, stressing that this is a test flight.
If the rocket is unable to take off Monday, September 2 and 5 have been penciled in as alternative flight dates.
One of the primary objectives of the mission is to test the capsule's heat shield, which at 16 feet in diameter is the largest ever built.
On its return to the Earth's atmosphere, the heat shield will have to withstand a speed of 25,000 miles per hour and a temperature of 5,000 degrees Fahrenheit (2,760 degrees Celsius) -- or half as hot as the Sun.
Dummies fitted with sensors will take the place of real crew members, recording acceleration, vibration and radiation levels.
A complete failure would be devastating for a program costing $4.1 billion per launch and that is already years behind schedule.
Monday's launch is "not a near-term sprint, but a long-term marathon to bring the solar system and beyond into our sphere," said Bhavya Lal, NASA associate administrator for technology, policy and strategy.
The next mission, Artemis 2, will take astronauts into orbit around the Moon without landing on its surface. The crew of Artemis 3 is to land on the Moon in 2025 at the earliest.
And since humans have already visited the Moon, Artemis has its sights set on another lofty goal -- a crewed mission to Mars.
The Artemis program is to establish a lasting human presence on the Moon with an orbiting space station known as Gateway and a base on the surface.
Gateway would serve as a staging and refueling station for a voyage to Mars that would take a minimum of several months.
