I found this response by one of their tax experts on another message board. I had the same issue and this really helped me:
"This is a known software issue. The workaround is to delete the $300 in the error check. Then go back to Deductions & Credits and enter the contribution there, in the Donations to Charity topic. Then 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 will be back on Form 1040 line 10b, but it won't be flagged as an error. It only gets flagged if you enter it when prompted after you finish Deductions & Credits and the software tells you that you are getting the standard deduction. It will not get flagged if you enter it in Donations to Charity."
