One of the "shades," later revealed to be Beatrice, first confronts Dante with his primary sin in Canto II. She tells him that his soul has been "assailed by cowardice" (line 45).
In lines 88-91, she admonishes Dante, saying, "One ought to be afraid of nothing other/than things possessed of power to do us harm,/but things innocuous need not be feared."
Still, Beatrice is not done with her chastisement, hoping that Dante will come around. In lines 121-126, she pleads with him: "What is it then? Why, why do you resist?/Why does your heart hold so much cowardice?/Where are you daring and your openness/as long as there are three such blessed women/concerned for you within the court of Heaven/and my words promise you so great a good?"
The sin, then, is storing up earthly treasures in lieu of heavenly ones, out of fear of what may become of us in the present. Cowardice, like lust, avarice, and all of the other deadly sins, will have its ultimate price.