If you disagree with the notice, call the IRS at the toll-free number on the top right corner of your notice. Please have your paperwork (such as cancelled checks, amended return, etc.) ready when you call. See also Publication 5, Your Appeal Rights and How to Prepare a Protest If You Don’t Agree.
Being proactive in addressing the SRP liability may prevent additional interest charges and eliminate the need for the IRS to apply future tax refunds toward the balance.
If you are able to full pay the balance owed, see for the various ways you can pay your IRS debt.
If you can’t pay the full amount by that date, you can consider payment options that might work for your situation, and contact the IRS to set up a payment plan or discuss other ways to address your balance.