Deductions & credits

If it won't let you just uncheck the box then you need to clear browser data.
 

Use the Chrome browser
Click the three dots in the upper right corner
Select Settings

Privacy and Security

Clear browser data for all time

Then sign in to TurboTax on the Chrome browser
Go to Personal Information - Review/Edit

Select No for Someone else can claim me as a dependent.

Any person who can be claimed as a dependent by another person is not eligible for the stimulus, even if the other person does not claim them.