I'm taking the standard deduction and entered $300 in cash Charitable contributions on form 1040 item b but I get a flag on turbo tax that the line should be blank.

Hoping this helps someone. 

I noticed, too, that Turbo Tax was not correctly filling in Line 10b on the 1040 based on the fact that I was not itemizing deductions but had $300 of charitable contributions.   I called tech support and the rep told me there was an interview question regarding the $300 and the question would also appear during review time.   I couldn't find any interview question.  But, the rep was correct in that during the Federal Review process, the software did flag an error and I had an opportunity to enter $300 during the error correction process.  By doing this, it filled in line 10b on the 1040 as well as the 1040 worksheet.   It worked.  But there should be simple logic that says if the filer didn't itemize and the software sees cash charitable contributions, it should put this amount (up to $300) on line 10b automatically without the fuss of doing this during the final review where the software flags on error.