Before the trip starts, why does the time travel company try to scare Eckles?

In the time travel company's defense, it probably tries to scare every patron that they work with; however, Eckels comes across to readers as deserving to be scared. He's overly confident in himself, and he believes that his money makes him invulnerable. He's arrogant and cocky, and the time travel company knows that can be trouble. They want Eckels to understand that what he is doing is reality. If he messes up, he dies. They need him to take the situation seriously, because his life is on the line. Additionally, their reputation as a time travel company is on the line. If Eckels dies while on their safari, that doesn't give the company a very positive image. Finally, they do not want his brash behavior getting the time travel company in legal or financial trouble. The company makes great efforts to ensure that nothing in the past is disturbed other than which is supposed to die at that time. If Eckels makes some kind of overconfident decision that messes up the time stream, then the time safari company gets in big trouble. They need him to be a little scared, so he listens to directions and does what he's supposed to do.