Short version. Verify that your charitable donations section is completely blank and didn't autofill from other years/areas, or that your totals match it. Itemize your charitable donations, even if you're taking the standard deduction, and then use that total, or $300, whichever is less.
Longer Explanation
I spent some time on the phone with a Turbotax employee before I figured this out. My issue was I had charitable donations that were itemized automatically from my 2019 state return ($17, it was a tiny return so we just donated all of it). When the system itemized these automatically I didn't know because I honestly had forgotten about them and knew I hadn't made sufficient charitable donations to matter above the standard deduction. I never opened that section this year at all. When the special CARES question about charitable donations came I did mark down the charitable donations I had an easy way to track from an online source, which was around $125. So I entered that. I then kept getting the error message saying that was too large. However when we blanked the box completely and came back to look at it the number 17 kept appearing. Finally when I went to look at itemized charity donations it had the donations from my state returns last year. That explained the random 17 that kept showing up. When I added my other 125 to it as an itemized donation and closed it I still got an error saying the line wasn't right. However this time I added the appropriate total (142) and that solved the problem.
