It wont let me add my 529 contributions because I didn't have any taxable imcome for prepaid tuition on my federal return? I contributed $6800 in 2018.

There is no federal deduction for that; so there is no where to enter it, in the federal section of TurboTax.

 Some states allow a deduction on the state return. If your state is one, it will come up in the state interview. For a list, see:

http://www.savingforcollege.com/compare_529_plans/index.php?page=compare_plan_questions&plan_question_ids[]=437&order_by=plan_question_437