1040, standard deduction, and $300 on line 12b. Final Review shows one error (screenshot attached).

No, you can't E-file if your return has an error. 

To reap tax benefits for making charitable donations, you must generally itemize your deductions rather than taking the standard deduction.

For 2021, if you’re claiming the standard deduction, this amount is up to $600 per tax return for those filing married filing jointly and $300 for other filing statuses.

You can correct your return(s) by logging back into your TurboTax program and following the steps below.

1. Click on the Federal Taxes tab.
2. Select the Deductions & Credits tab.
3. On the screen titled How do you want to enter your deductions and credits?, select I'll choose what I work on.
4. Scroll down to Charitable Donations.
5. Select Update to the right of Donations to Charity in 2021.
6. You will see a screen titled Review All Your Charities.
7. To the right of any deductions entered, select Delete to remove any input.
8. You will then select Done with Charitable Donations.
9. On the next page (Deductions), scroll down to the bottom and click on Done with Deductions.
10. Proceed through the following screens until you pass the screen letting you know you are getting the Standard Deduction.  
11. The next screen titled Charitable Cash Contributions under the Care Act will allow you to enter your charitable contributions for 2021.  Enter your total cash contributions on this screen and select Continue.

Source: JotikaT2 Employee Tax Expert

Thanks for the reply. 

I tried the fix, with no success .... there is no entry to delete in the donation list, and the error just wouldn't go away.

Eventually, I tried the checkbox "Do print as support doc" (something to that effect) when zooming in from line 12 in Schedule A.   And right away, the pink field and red zero in line 12 disappeared.   No more error during Review.