Starting/igniting a liquid fueled rocket engine is an inherently complex process - everything has to be sequenced just right to get engines chilled, turbo pumps up to speed, any gaseous fuel vented and harmlessly ignited before it builds up, ignition of fuel, etc.

Here's a 1hr video from the Everyday Astronaut explaining the process and everything that can go wrong.

https://www.youtube.com/watch?v=bAUVCn_jw5I