Cancellation of Debt

I received a notice from the IRS for changes to my 2018 tax returns that listed 2 Cancellation of Debts. Both were unsecured debts in which I have worked with a debt settlement company to pay off these debts. Both have been settled and paid off and I was not aware that this was going to happen. The total for the 2 Cancellations of Debts are $7,787 and the notice is for a “Proposed Amount Due of $2,015” and should be paid by 12/30/2020. The notice gives me options to respond if I agree or disagree. This is the first I’ve heard of this, unfortunately the debt settlement company did not disclose to me that this could happen. What options do I have if I do not agree with the change in my returns? Most importantly, is there an option to not have to pay this? Thank you I’m advance for any guidance you can share.