You have not already filed the 2020 tax return, have you?
On my online edition I can click on Federal Review and it gives the screen Let's make sure you got the right stimulus amount and then clicking on Continue I have the page for the stimulus questions to enter amounts.
If you cannot access the stimulus page then contact TurboTax support for assistance.
Use this website to contact TurboTax support during business hours - https://support.turbotax.intuit.com/contact/
Use this phone number and select TurboTax - 1-800-4-INTUIT (1-800-446-8848)