It doesn't matter if you use the standard deduction or itemized deductions. When you enter your Charitable Contributions, TurboTax automatically deducts $300 from your Charitable itemized deductions and carries them to the front of Form 1040. This is generally more beneficial to you. 

For 2020, there's a special provision on your federal income tax return. Even if you don’t itemize, you may take a charitable deduction of up to $300 for cash contributions made in 2020 to qualified organizations.

•  Under this new change, individual taxpayers can claim an "above-the-line" deduction of up to $300 for cash donations made to charity during 2020.
• This means the deduction lowers both adjusted gross income and taxable income – translating into tax savings for those making donations to qualifying tax-exempt organizations.

I have entered $300 on 10b and keep getting error message

I recommend following the steps below to ensure your return is populated correctly.

Review your entry in the Federal interview section of the program.

1. Select Deductions & Credits
2. Scroll down to All Tax Breaks and select Charitable Donations
3. Select Start to the right of Donations to Charity in 2020
4. Proceed to enter the details of your charitable cash contribution by entity as applicable in this section.
5. Once you have completed entering your details, select Done with Charitable Donations

Continue through the screens until you see the screen titled Charitable Cash Contributions under Cares Act.  You will be able to confirm the amount of your cash contributions on this screen.  (This screen will only show up if you are taking the standard deduction.)

Your input in both of the sections above will ensure you are either able to take the deduction if you are able to itemize, or will pick up the adjustment if you are taking the standard deduction.

In the past, you were only able to claim a credit for donations if you itemized your deductions. 

However, in 2020, you are able to take a $300 above the line adjustment for cash contributions made to a qualifying charity.

See the following link for more information.

Cash charitable donations information