I received a letter from the IRS saying I owe another $21378 from my 2017 taxes. I used turbotax and entered everything. Do I have any recourse other than to pay it?

[Critter]

First review the notice and your return ... is the IRS correct ?   What did they change or say is missing ?  

[VolvoGirl]

What is the letter number up in the corner?  The CP or LTR#?  Be careful, it might be a scam.  I would call the IRS and double check.  Did you get a big refund for 2017?  The letter should say what they changed.