Schedule SE-T error on federal review tells me to enter line 18,thought turbo tax does this on its own?

Line 18 isn't automatically filled out because it calls for you to make an estimation.  Only you have an idea of what the amount might be.  

See the instructions for Line 18 of Schedule SE in the IRS's 2020 Instructions for Schedule SE for guidance on what you need to enter.  The instructions are very vague, but also open-ended, allowing you to use any reasonable method:

Use any reasonable method to divide the net profit or (loss) you reported on line 3 between (a) January 1, 2020, through March 26, 2020, and (b) March 27, 2020, through December 31, 2020. A reasonable method will accurately reflect the time when income was earned or accrued and deductible expenses were paid or incurred, depending on your method of accounting. In most cases, a proportional division of the amount reported on line 3 based upon the number of days in each period will be considered reasonable.

In the case of a loss for 2020, the software seems to have a looping error.  In my case, I had a loss for 2020. When I enter a negative number for Sch SE Line 18, it says "Minimum Value is 0".  If I enter "0" or anything greater, the software says "Max deferral Line 18 is too large."  So there literally appears to be no value that the software will accept, and without fixing the problem, I can't e-file.  Any solutions out there?

Why doesn't the interview ask me to enter a value rather than surprising me with an error during the review?