Well... if you efiled the return AND you itemized then the Sch A would have been included in the return transmitted.
Now if you mailed in the return, you may have not attached the Sch A or the IRS lost it ... print one out and send it as instructed in the notice they sent.
The ONLINE system has permanently closed and is presently in transition from 2018 to 2019 versions.
While the millions of returns are being archived the PDF download for tax year 2018 will only include the forms and schedules needed to file your tax return. The worksheets and other pages are not included in the PDF until the 2019 online editions become available sometime in December 2019.
To access the PDF, log into the website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)
For 2018? How much is on your 1040 line 8? Maybe you took the Standard Deduction. The Standard Deduction doubled so more people took the Standard Deduction instead of itemizing this year.