Sorry no. There is no amend software for a Mac.
You will need to contact TurboTax support and request that they push a 2018 Mac installation file to your download account so that you can amend the 2018 tax return completed using the online editions.
Use this website to contact TurboTax support during business hours - https://support.turbotax.intuit.com/contact/
Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax
