Standard Deduction-Charitable expense deduction For a married couple the "Check your return feature" keeps reducing the max charitable amount from $600 to $300.;

Please try these steps to enter the charitable cash contribution for 12b:

1. Open you return
2. Click "Federal"
3. Click “Deductions &Credits” on the top
4. Click "I'll choose what to work on" (do not enter any donations under Charitable donations)
5. Scroll all the way down and click the "Done with deductions" button
6. Continue after the "The Standard Deduction is right for you!" screen
7. Continue until the "Get a tax break for donating cash to charity" screen
8. Enter your charitable cash contributions (do not enter more than $600!)

Please see Why am I getting an error message after entering charitable cash donations  for additional information.

This has been an issue reported by a number of users.

Make sure your version is up-to-date as TT is constantly making updates.  Supposedly it is fixed, but no promises.

See the response below from a TT lounge discussion thread:

If a charitable donation of cash is entered on Schedule A the program still comes up with an error for the entry of $300 on the 1040 Line 12b when filing as Married Filing Jointly but entering $600 corrects the problem and the error does not re-occur unless you go back and add data to the federal return, then in Final Review the error pops up again.  Correcting to $600 fixes the problem again.

Yes, your solution worked!