Requested the MAC software to amend my 2016 taxes. Turbo emailed a link with a code and password. End response... "combination is invalid or has already been claimed."

This happened to my office mate as well.  Never got a MAC one to work.  We managed to work around it by using a Windows PC to download and run the 2016 amending program.

Contact customer support:

Here is a TurboTax FAQ for contacting customer support.

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I note that he did contact customer support two or three times before we resorted to Plan B.