IRS flagged me for not reporting the sale of my home in 2018

I used TurboTax Deluxe to file my 2018 taxes.  Also in 2018 I sold my home.

I just received a letter from the IRS saying I did not report the sale, and therefore there is a change to my taxable income.  So now the IRS says I owe thousands of dollars!

I checked my 2018 tax returns, and sure enough I do not see the sale.  First, I want to be clear, I did not intentionally avoid reporting it.  I must have just missed the question, or maybe went down a path of questions that did not include asking me.  In any case, I will correct whatever needs correcting.

Second, I thought my home sale profit would be excluded from taxable income because I lived in it (primary residence) from 2011 to 2018, I'm married and filed a joint return, and the profit did not exceed $500,000.

Any insight/advice would be greatly appreciated!