There was zero fee to file my taxes and now that I want to amend the previous year it won't let me. What would be the best option

[Kat]

For TurboTax Free Edition filers, you'll be required to add the PLUS benefit in order to amend your prior-year return.  Here's a link to instructions on how to add the PLUS benefit:

How do I add PLUS benefits? 

[macuser_22]

If you mean 2020 tax then that cannot be done online anymore so Plus will not help.

2019 and earlier can only be amended with TurboTax using the CD/download version installed on a PC or Mac computer.

2019 amended Federal returns can either be e-filed or mailed. 2018 and earlier amended returns can only be mailed. It is suggested that it be mailed certified with return receipt (or other tracking service) to verify that the IRS or state receives it.

See this TurboTax FAQ for detailed amend instructions:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

-- Amended returns can only be mailed - allow 8-12 weeks - can take up to 16 weeks (4 months) for processing.

You can check the status of the amended return here, but allow 3 weeks after mailing.

https://www.irs.gov/filing/wheres-my-amended-return