Ignore line 12 for now. How much is the standard deduction on line 9 of your original 2019 Form 1040 (not the 1040X)? If your income was high enough, it's possible that the standard deduction you got as a dependent is the same as you would get when you're not a dependent. If the standard deduction is $12,200, it won't change. In that case, saying that you cannot be claimed as a dependent doesn't change anything on your tax return, and there is no reason to file an amended return.
If there is no change, and you are filing this amended return to try to get a stimulus payment, it's too late for that. You will get your stimulus payment in the form of the Recovery Rebate Credit on your 2020 tax return.
If your standard deduction on the original return was less than $12,200, it should be increased to that amount. Did you uncheck the box in the My Info section that says someone can claim you as a dependent?
