You are using the TurboTax Deluxe online edition so you do not have to pay anything for support.
See this TurboTax support FAQ on how to contact Support - https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-contact-turbotax/00/26991
For the TurboTax desktop editions -
To remove the error I went into Forms mode and deleted the Charitable Organization Worksheet. Then on the Form 1040 Worksheet for Line 12b removed the $300 amount so Line 12b is now blank (empty).
Going back to Step-by-Step I completed the Deductions & Credits section without entering anything for a charitable donation. Continuing through the screens until landing on the screen labeled Charitable Cash Contributions under Cares Act, I entered $600. Continued through Final Review without error.
For the TurboTax online editions -
The problem appears to be that a charitable donation was entered in the Deductions and Credits section of the program when you are only eligible for the Standard Deduction.
Click on Tax Tools on the left side of the online program screen
Click on Tools
Click on Delete a form
Scroll through the all the forms, schedules and worksheets listed and delete all the Charitable Organization forms listed.
When completed with removing forms click on
Continue With My Return
at the bottom of the list.
Click on Federal on the left side of the screen
Click on Deductions & Credits at the top of the screen
Scroll down to the bottom of the page and click on
Wrap up tax breaks
Continue through the screens. After the screen showing that you are getting the Standard Deduction the following screen is Charitable Cash Contributions under Cares Act.
Enter $600 if filing Jointly or $300 for all other filing status'. The federal review and the Final Review should no longer show an error for the charitable donation.