Why won't TurboTax accept my charitable contributions value?

If this is concerning the "up to 300 above the line deduction,

and you are using TurboTax Desktop,

you need to switch to step-by-step (on top right) first 

DESKTOP

Go to Step-By-Step input

Click Federal

Click Deductions & Credits along the top

Click “I’ll choose what I work on”

Edit or Update Donations to Charity in 2020 if needed, then click “Done with Charitable Donations”

Scroll down the “Your 2020 Deductions & Credits” screen and click “Done with Deductions”

Click Continue on the “Let’s Check Your Deductions and Credits” screen

Continue through the following interview questions until you get to the “Here Are Your 2020 Deductions & Credits” screen

Click Continue

If the next screen says “The Standard deduction is Right for You!” click Continue

ON THE NEXT SCREEN enter the cash contribution made in 2020 and click Continue

Your 1040 should update

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.) “