The blank line 12c on Form 1040-SR is a bug. It has been reported to TurboTax.
You didn't say whether you are using TurboTax Online or the CD/Download TurboTax software. If you are using TurboTax Online there is nothing you can do except wait for TurboTax to fix the bug.
If you are using the CD/Download software, you can make it print Form 1040 instead of Form 1040-SR. Form 1040 will show the correct amount on line 12c. In forms mode, open the Federal Information Worksheet. Scroll down a little to Part II, where your address is shown. A few lines below your address is a line that says "Print Form 1040-SR instead of Form 1040." On that line, click the No box. Then it will print Form 1040.
