Why aren't my cash donations showing up when I enter them?

Please try the following workaround: 

1. Delete the $300 in the error check.
2. Then go back to Deductions & Credits and enter the contribution there, in the Donations to Charity topic.
3. Click Done with Donations and proceed through the screens until you get to the screen that tells you that TurboTax has added in the contribution.

The contribution should appear on Form 1040 line 10b.