The only tax you can defer is part of the self-employment tax, and only under certain circumstances. If you don't have a balance due on your tax return (i.e. you are getting a refund) you probably cannot defer any tax. TurboTax will ask you how much of your self-employment income was earned from March 27 to the end of the year. It will then calculate the maximum amount you can defer and ask you how much you want to defer.
I'm not sure how you get to this in Self-Employed Online. It's probably in the Personal section under Deductions & Credits > Tax relief related to COVID-19 > Self-employment tax deferral. If you have trouble finding it, a TurboTax Live tax expert might be able to help.
Note that the first half of the deferred self-employment tax has to be paid by December 31, 2021.