underestimate or overestimate?
You cannot underestimate …
The sentence is usually something like this:
You cannot underestimate the effect of luck on success in life.
What is meant is that luck is a powerful factor in the achievement of success, so powerful that you cannot speak too lavishly or extravagantly about the force it wields. That is to say, you cannot overestimate the effect of luck on success in life. In this instance it is impossible to go over the top as you ramp up your estimation of its influence.
Perhaps the confusion is with the idea that you really, really ought not to underestimate the effect. I don’t know, but somehow the phrase slips out and the mental calculation on the force of the negative with the following verb is never made. We really should avoid the phrase if we are not prepared to think it through.