You must be using TurboTax Online.
TurboTax Online 2020 is seriously flawed (see other posts on the forum about this).
It won't let you change/correct 1040-X line 18.
To get a completely correct Form 1040-X you'd have to fill out the form manually and file on paper.
you can get 1040-X in pdf-fillable format from IRS.gov.
