If your 2016 federal income tax return was audited by the IRS, you should have very little doubt about what you owe the IRS. You should have been provided with a Notice of Deficiency, sometimes called a 90-day letter outlining the results of the audit and the tax, interest, and penalties that you owe. The IRS Collection Division has never been particularly reticent about following up on these notices.
Your next steps should be the following:
1. If you have not filed your 2017 tax return, do so now, even if you cannot pay any tax due.
2. Timely file your 2018 tax return. It is due by April 15, 2019. You may file for an automatic six month extension to file, but this is not an extension of time to pay.
3. Pay, or make arrangements to pay, the balance from your 2016 Notice of Deficiency.