The Associated Press (AP) has reported that Boeing aborted its first astronaut launch due to a problem with the rocket's valve.

According to Amac News: Boeing said two NASA test pilots had just been strapped into Boeing's Starliner capsule to fly to the International Space Station when the countdown stopped.
United Launch Alliance CEO Tori Bruno said an oxygen depressurization valve on the Atlas rocket's upper stage began to open and close, creating a loud noise.
The CEO of United Launch Alliance said that this valve may have been rotated 200,000 times and thus worn out.
He added that if the problem is resolved soon, the launch team could try to launch the astronaut again on Tuesday night.
It was the latest delay for Boeing's first crewed flight, which has been stalled for years by capsule problems.