I am filing single, standard deduction, why won't it allow me to take extra deduction for cash donations?

Don't use decimal points, only enter whole dollars. 

The review screen should allow you to enter the number. 

You may also find these instructions helpful: 

TurboTax ONLINE

Click Tax Tools on the left side bar

Click Tools from the drop-down

Click the "Topic Search" box on the screen

Type or scroll down to    standard deduction and exemptions

Click GO 

The next screen gives you an option to change your selection from Standard to Itemized and back. You do not need to change your selection

Scroll down to the bottom of that screen and click CONTINUE

The next screen is the Charitable Cash Contributions under Cares Act screen

IF YOU ARE CLAIMING STANDARD DEDUCTION enter the amount of cash contributions you made, up to  300

IF YOU ARE ITEMIZING DEDUCTIONS enter 0

Scroll down and click Continue

Click Continue on the next screen

“The new "up to $300 above the line cash charity donation" is available for tax year 2020. This allows a Taxpayer to claim up to a $300 credit which is subtracted from income on the 1040 line 10c if the Taxpayer is claiming the Standard Deduction. (If you Itemize Deductions, all your donations will be listed on Schedule A.) “