Are you referring to Line 319 - Interest paid on your student loans on Schedule 1?

In TurboTax Online 2017, select “Students” from the “Deductions” tab. Check off “Student loan interest paid in 2017 or not claimed in a previous year” and click the blue “Continue” button. You may have to go past a few pages, but you should eventually get to the “Student Loan Interest” page. There you can put in your current and past year’s unclaimed interest.

In TurboTax Online 2018, select Deductions>Students>Student Profile from the left side menu. Check off “Student loan interest paid in 2018 or not claimed in a previous year” and click the blue “Continue” button. You may have to go past a few pages, but you should eventually get to the “Student Loan Interest” page. There you can put in your current and past year’s unclaimed interest.