The launch of the new SLS rocket for the mission Artemis I was scrubbed earlier today due to valve issues.  Rocket engines, especially multi engine systems are very complex and dynamic systems in which any sign of a problem means the best policy is to abort.  Especially before launch.  These engines run on hydrogen and oxygen which must be kept at near absolute zero Kelvin.  As such an engine being the wrong temperature can indicate a leak.  

Furthermore, a hydrogen leak, can mean a possible hydrogen fire.  Hydrogen burns with a flame that would be invisible to the naked eye.  It has the highest energy density of any fuel, it is cheap and plentiful but hard to handle.  NASA was quite right to make sure the multi billion dollar SLS rocket did not have a catastrophic failure and abort when they did.  

Watch below for the words of people who know way more about this than I do . 
From a scientist mindset this is not a "failure".  NASA has learned something new about how this vehicle will behave up to and almost including an actual launch.  Even if it had launched, went up almost to orbit, then exploded the telemetry from it would've taught us what went wrong and how to do it better next time. 

A brief word on politics.

From a political perspective it would've just been a time for Rand Paul and or Alexandria Ocassio Cortez to Tweet about what a waste of money SLS and perhaps NASA itself are.  So it is crucial that Artemis I not fail.  It would lead to bipartisan, broad spectrum, support for cutting NASA's budget. One thing non US people do not often get about the United States of America is just how powerful the congress of the United States is.  We may hail the chief and treat the president as very powerful.  In our foreign relations they are.  Internally Congress holds power like that once held by an absolute monarch.   What they vote out of existence, legally, ceases to exist and vice versa.   A vote on the house and senate floors could end NASA in as little as a day if there were broad agreement. 

A failure by SLS, a spectacular explosion on the launch pad would've been the end of it.