Thank you for the additional information and the screenshot. You are correct, that is what the form should look like. The error would occur if you entered anything in the Cares Act section.
TurboTax is considering it an override. Try leaving this field blank and running the review again. This will clear the error. TurboTax wants to calculate it for you. It will still carry $300 to Line 10b.
For other details about Charitable Contributions, see: Charitable Contribution Deductions | Internal Revenue Service
