I forgot to list a debt in my Chapter 7 Bankruptcy and now the case has been discharged and closed! Can I still list the debt?

So long as it was a "no-asset" chapter 7 case and you did not intentionally leave the creditor off, it is probably discharged anyway, even without listing it. According to a court case from Houston in 2013, even unscheduled debts are discharged in a no-asset chapter 7 case, unless the creditor can prove that the debtor left off the debt fraudulently. The Court held:

A Chapter 7 discharge grants a debtor a discharge under § 727. Section 727(b) declares that: "[e]cept as provided in section 523 of this title, a discharge under [§ 727(a)] discharges the debtor from all debts that arose before the date of the order for relief . . . whether or not a proof of claim based on any such debt or liability is filed." (emphasis added). Thus, unless excepted by § 523, a creditor's pre-petition claim is discharged. E.g., In re Guzman, 130 B.R. 489, 490-91 (Bankr. W.D. Tex. 1991) (finding that § 727 discharges all pre-petition debts, but that creditors may attempt to prove an exception under § 523).

But please make every effort to list all of your creditors. You are swearing under penalty of perjury that you are listing all of your creditors, and if you intentionally leave one off, you likely will never be able to discharge it in bankruptcy.