I got the charitable contributions error and typed a value into line 12b. Now I need to correct it - but how? (changing donations list doesn't overwrite that value)

If you are trying to get the additional $300/$600 deduction added to the standard deduction, this is the process.

First, you must clear all donations from the Deductions & Credits input screen

Online version

1. Go to Deductions & Credits

2. Scroll to the very bottom

3. Select Wrap up Tax Breaks (Online)

4. Continue through the interview to 

5. Charitable Cash Contributions under Cares Act

6. Enter your donation.

In the Desktop version:

1. Go to Deductions and Credits

2. Scroll to the bottom of the page

3. Select Done with Deductions

4. Let's check your Deductions .. Continue

5. Here are your 2021 Deductions ... Continue

6. The standard deduction is right for you ... Continue

7. GET A TAX BREAK FOR DONATING CASH TO CHARITY

8. Enter your amount