Anyone who checks the box on the tax return that they can be claimed as a dependent by someone else is not eligible for the stimulus payment. If the person meets the requirements to be claimed by someone else, they must check the box even if nobody else claims them.
If you do not meet the requirements to be claimed by someone else and checked the box by mistake, you will have to prepare, print and mail an amended return form 1040X to "uncheck" the box. DO NOT uncheck it until you have started the amended return. Here is a link to help you: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
An amended return cannot be efiled. You must print and mail it yourself. TurboTax does no mail it for you. In normal times, the IRS takes about 16 weeks to process an amended return.
Since the stimulus payment is an advance credit on 2020 taxes, your alternative is to wait until you file your 2020 tax return early next year and indicate you cannot be claimed as a dependent. You would get the stimulus payment as a credit on that tax return.
