CP2000 received for 2018

You should have filed Form 8889 in which you would have reported that your HSA withdrawals were for qualified medical expenses in which case they would not be taxed. 

No, HSA distributions for medical expenses are not taxable.

The IRS received a copy of form 1099-SA which reported the distributions from your HSA (you should have gotten a copy, too). If you would have entered this form in TurboTax, it would have generated a form 8889, which, as Bsch4477 notes, is required to show that your distributions were all for medical expenses.