Prestige unlocks at 5 billion lifetime Crystals in the current run. At 1 billion, you are about 20% of the way there.
That said, I checked the game and the prestige panel currently only starts showing later, around 1.75 billion Crystals, so the requirement is not clear enough. I’m going to improve this in the next patch so Prestige is visible earlier and clearly shows your progress toward 5 billion.
Thanks for pointing this out — this is a real clarity issue.