Can anyone explain why, when I entered the $300 charitable contribution as allowed, am I getting an error that it is too high?

Entering this deduction is a two-step process...please see below:

For 2020, Congress has changed tax law to permit the deduction of certain charitable contributions “above the line”, even if the taxpayer is taking the Standard Deduction otherwise. The new deduction will appear on line 10b on the 1040.

There are restrictions: the contribution must be in cash or cash-equivalent and it cannot exceed $300 for most filing statuses (exception: $150 for Married Filing Separate).

To enter this deduction, you must enter the charitable contributions as if you were going to itemize these contributions (do a Search (upper right) for charitable contributions and click on the jump-to result). Be sure to indicate that the contribution was “Money” or else TurboTax will not ask about it later.

Note that you do not need to enter any other charitable contributions if you are not planning to itemize.

Then, in the Federal Review at the end of your data entry, TurboTax will realize that you are taking the Standard Deduction and ask you to enter the amount of the charitable contribution that was in cash. Note the limits described above ($300 or $150); in the desktop product, at least, if you enter an amount larger than this, you will be asked to reduce it to the limit when you enter the amount.

When you enter the cash-based charitable contribution this way, the result will appear on line 10b on the 1040 as an above-the-line deduction.