If you are using a CD/downloaded version of TurboTax, please be sure that you have updated the program to the latest version. Do this by clicking Online > Check for Updates in the top left section of the screen.
Then, be sure that you have entered your cash contributions in the Charitable Contributions section of your return. If you have entered at least $600 of cash contributions and you are using the Married Filing Joint filing status, then line 12b should show $600.
If you find that yours is still incorrect after updating the program or if you are using TurboTax Online, please respond back and let us know.
I am using desktop & I upgrade the program everytime before I use it. I even looked at my 2020 return & it had done the same thing. I manually corrected it in order to file 2021 return but it should be corrected.
I have entered over $600 in the contribution section but it still only carries $300 over to line 12b.
Are you using the 1040SR? Switch to the 1040 then enter the $600.
@acctcompconsult Please visit the link below for a helpful solution posted by ColeenD3.