10a from schedule 1, line 22 I entered 300 (which is $100 less than donated) but says 300 is max

Yes ,this year you can claim $300 as an adjustment to income (line 10b) if you made a cash contributions to a charitable organization, while claiming the Standard Deduction.

If you see the $300 on line 10b then it seems you entered everything correctly.

Please follow these steps to enter the charitable cash contribution for 10b (do not enter it under Deduction & Credit, Charitable Donations) :

1. Login to your TurboTax Account 
2. Click "Federal" from the left side of your screen
3. Click “Deductions &Credits” on the top
4. Scroll all the way down and click the "Wrap up tax breaks" button
5. Continue until the "Charitable Cash Contributions under Cares Act" screen
6. Enter your charitable cash contributions.

To verify your entry:

1. Click on "Tax Tools" in the left menu
2. Click "Tools"
3. Click "View Tax Summary" in the Tool Center window
4. Click on "Preview my 1040" on the left