I was having the same issue...only difference is that I'm using the desktop version, not online.
In the questionnaire, it asked if I wanted to defer my SE tax payment until 2021, I clicked Yes to find out more info, but went back and answered No as I decided to just pay it now since I'm receiving a refund.
In the error check, it returned a result of Sch SE-T: max deferral line 18 must be entered (which is the net profit between 3/27 and 12/31 as shown on Sch C line 3...I had to go directly to form Sch SE-T to find this info). I went back to the questionnaire and selected No again for deferment, but the error was still there. So, I figured out that amount to see what would happen, and obviously it deferred a percentage of my payment until next year...which I did not want to do.
After talking with a TT Rep, we concluded the fix was to put in $0 as that amount (either in the questionnaire or directly in the form), which then returned no errors during review. Even though that is not the correct value, it was the only way to override the error and to not defer a portion of my payment. I'm assuming the IRS won't care as they are getting my full SE tax payment now.
I feel this is somewhat of a bug in that choosing No in the questionnaire doesn't override a previous selection of Yes (should you go back to the question) or that the error check doesn't full explain what you are doing by inputting this amount, and give you an option to not defer payment so as to fix the error.
It seems like you should be able to go directly to the Sch SE-T form for the online version at anytime to change the line 18 amount, but I'm not familiar with that interface. If it's not possible given the point you are at, this may not help you, but may help others that come across this post.
