If you make the manual entry you will still be able to e-file. However, the instructions that you quoted leave out some important details.
First of all, you have to make the manual entry in forms mode, which means you have to use the CD/Download TurboTax software. TurboTax Online does not have forms mode.
Secondly, you can't actually make the entry on Schedule 3 itself. In forms mode you have to open the "Form 1040 or Form 1040-SR Worksheet." Scroll down to the Schedule 3 section of the worksheet, and then to Part II, Other Payments and Refundable Credits. It's almost at the end of the very large worksheet. In Part II, between lines 10 and 11, there is a big box titled "Excess Social Security and Tier I RRTA Tax Withheld Smart Worksheet." That's where you enter the excess amount that you calculated. The amount you enter there will then appear on Schedule 3 line 11.
