You would be better off amending with TaxAct because they already have your 2018 tax return as a starting point.
To amend using TurboTax, you must first purchase the 2018 CD/download software and install that on a PC or Mac computer. Then reproduce the 2018 tax return for filing *exactly* as it was filed - errors and all. Only then can you start the amend process, which can only be printed and mailed.