A short in a switchbox or an electrical line appears to be the reason one of the  heaters for the fuel line powering one of Endeavour's three auxiliary power units failed.   The heaters keep the hydrazine from freezing on orbit.  Thus, the mission has been scrubbed.

The power units provide hydraulic pressure to the main engines at liftoff and to the rudder and speed brake during landing and for redundancy all needed to be functional in order to give the go-ahead for Endeavour,  the shuttle built to replace the Challenger shuttle destroyed during liftoff in 1986, to return to the International Space Station for its last mission in its 19-year history. 

They say the launch could be Monday but that was not certain.   The six astronauts,  led commander Mark Kelly, were on their way to the launch pad when the countdown was halted.    
700,000 people were expected watch the launch live.  After Endeavour, only one flight by Atlantis remains before NASA ends the 30-year-old program and retires the fleet to museums.

The scientific purpose of the mission is to deliver the Alpha Magnetic Spectrometer (AMS) - and to put some squid in space.