Prior to the update downloaded today (18 Jan 21) everything I had checked seemed to be OK. Now with the update there is no entry on line 15 and all of my withholding is showing a refund due. Since my data didn't change I have to suspect the it the TT software.
Line 15 would be your taxable income. It could be blank if you had deductions that reduced it to zero. I suggest you look at line 9, total income, and then at the lines below that to see what reduced your income.
You can view your form 1040 while working in the online version of TurboTax by following these steps:
While working on your return in the Federal section of TurboTax:
1. Choose the Tax Tools icon on your left menu bar
2. Tools
3. View Tax Summary
4. Choose the Preview my 1040 on your left menu bar
Everything is correct down through line 11, the Standard Deduction on 12a is correct for my status, and there is a $600 entry on line 12b, but there are no entries on lines 12c, 13, 14, or 15 (all of these are calcuated by TT). Perhaps somebody knows what the pre-conditions are for those entries to be calculated.
Solved. The box for line 12c on the 1040 Worksheet was empty but had Override checked.