No, you cannot use TurboTax Online to amend your 2018 tax return, but you can use TurboTax to do so. The online version is only for current year (2020) returns.
Take a look at the following TurboTax article for instructions to amend your 2018 return using TurboTax:
How do I amend a 2018 return in TurboTax?
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