In trying to determine AMT for 2019, turbotaxonline is asking for the "Adjustments or preferences treated as exclusion items" from the 2018 return. Where is that?

If you had any AMT preference items on your 2018 tax return, they would be on Form 6251 - Alternative Minimum Tax - Individuals.  If you prepared your tax return using TurboTax in 2018, if the program did not generate a Form 6251, you did not pay any AMT and would not have any preference items to carry forward.