The system keeps rejecting my 300$ charitable cash contribution as too large. in fact, it rejects 0$ and 150$. How do I fix this bug?

If you using the election to take a charitable deduction of up to $300, on top of using the standard deduction, you cannot enter this through the Itemized Deductions screens. 

Please follow the steps below to enter your charitable contribution in the correct place in TurboTax :

1. Open your return. 
2. Click Federal from the left side of your screen.
3. Click Deductions &Credits on the top of the screen.
4. Scroll down and click the Wrap up tax breaks button.
5. Continue until you reach the Charitable Cash Contributions under Cares Act screen.
6. Enter your charitable cash contributions.