You need to finish all 6 optional quests in the Prologue. These are all located in Candlekeep, the area where you start the game.
This is missable, since you can return to Candlekeep after you finish the prologue. Talking to Gorion and choosing the "Ready to Go" conversation option ends the prologue, so make sure you complete all of them before doing that.
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Firebead's Scroll-
Phlydia's Book-
Dreppin's Cow-
Reevor's Storehouse-
Hull's Sword-
An Errand for Fuller