How do you get back to the page that says to enter line 18 deferral amount?

Your self-employment income may be eligible for a portion of your self-employment tax to be deferred.  See this TurboTax Help.

IRS Form SE line 18 allows you to report that portion of your self-employment income can be attributed to the calendar period of March 27, 2020 through December 31, 2020.

In TurboTax Online Self-Employed, to enter self-employment tax deferral, follow these steps. 

• Down the left side of the screen, click on Federal.
• Across the top of the screen, click on Deductions  & credits.
• Under Your tax breaks, scroll down to Self-employment tax deferral.
• Click on Edit/Add to the right of Self-employment tax deferral.
• At the screen Do you want more time to pay your self-employment tax?, click Yes.
• At the screen Let's start by getting your eligible income, this dollar amount will be reported on IRS Form SE line 18.
• Click on Continue.