How can I enter numbers in to resolve the"Check This Entry" Sch SE-S: Max deferral line 18 "must be entered" error in review? It won't let me enter a number and continue.

In review of my Schedule SE-S for self-employment tax, TurboTax says I need to enter in the Max deferral line 18 (it's blank currently). I don't really know what this is but believe it's how much of your self-employed income you want to defer paying taxes on. I don't want to defer any of it so I would like to enter zero but doing so does nothing. I cannot continue through the review process.