I received a letter from IRS saying the tax returned filed is different from what IRS received. 1099 K form that they received - I never received this form.

you can go to IRS website and download a transcript for whatever year the notice concerns   or you can call the IRS at number in notice.  TT never receives any info from the IRS (or others for that matter) so it would not know why the IRS says you have a 1099K when you say you never received one  - could you have moved.    also check the year the notice is referring to.  it might not be 2018.    a 1099k is for    Payment Card and Third Party Network
Transactions - such as selling items on EBay etc.  

Should you have received a 1099K for your 2017 tax return?  See this from the IRS  https://www.irs.gov/businesses/understanding-your-1099-k

 Should you have received a 1099K for your 2017 tax return?  See this from the IRShttps://www.irs.gov/businesses/understanding-your-1099-k