Form 1040-X is not available when your return is done processing. It is usually not available until later in the tax season, which is typically the last week in March. Current year Form 1040-X can be e-filed in TurboTax.
For the most recent updates, check: IRS forms availability table for TurboTax individual (person) tax products.
Supposedly it was to be made available to anyone by 26 March.....but with the recent messy congressional and IRS actions, I wouldn't be surprised if TTX delayed it a few weeks longer...
No rush though, you can't stop/intercept what has already been filed anyhow..