$300 dollar charitable deduction not lowering the Federal taxes I owe.

So, my taxable income is (AGI) is about 45,000.

 

On my return claiming as single it says 12,550 + the 300 charitable deduction = 12,850.

 

I owe $780 in Federal taxes, but when I added the $300 charitable deduction I'm still at owing $780.

 

Why would that be?  :(