Reuben, the oldest brother, refers to what he had said when he warned his brothers in 37.21-22.

Sin against the lad: Reuben had warned them not to kill Joseph, because he planned to return him secretly to his father. Sin in this context means “harm,” “hurt,” “wrongdoing.”

But you would not listen: that is, “But you [plural] refused to do as I said.”

So now there comes a reckoning for his blood is literally “and now his blood is required.” This refers to the belief that the blood shed in a murder requires retribution or revenge—it is not possible to murder without being called to account. For the introduction of this concept, see 9.6. Reuben, who had warned the brothers not to harm Joseph, is now saying the equivalent of “Now we have to pay the price of his death,” “Now we have to pay for having killed him,” or “We will suffer now for what we did to him.” See Good News Translation.

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .