I'm not sure what you have tried but delete the cash contributions completely in the "Deductions & Credits" tab using the "Charitable Donations" "Donations to Charity" topic.
After deleting the amounts click done all the way through the deductions interview clicking "Wrap Up Tax Breaks" or "Done with Tax Breaks" to trigger the updates.
Next, revisit the "Charitable Donations" topic and enter your cash donation up to $300. Click "done" through the deductions interview clicking "Wrap Up Tax Breaks" or "Done with Tax Breaks" to trigger the updates. Continue again to get to the page that gives you the choice for the Itemized or Standard deduction. Continue.
You should arrive at a page to verify the Charitable donation with the standard deduction. Verify and continue.
The error should clear now.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
