Why can't I check the box to contribute to the Presidential Campaign?

Yes, you can check the box to contribute to the Presidential Campaign fund.

Here's how to find it in TurboTax Online:

1. Open your return.
2. Scroll down to Other Tax Situations, select the down arrow to the right. 
3. Select Edit or Pick up where you left off.
4. On the screen, We're almost there..., select the down arrow to the right of Other Return Info.
5. Select Start or Revisit to the right of Presidential Campaign Fund.
6. Answer Yes, to Do you want to designate $3 to a presidential campaign fund?
7. Select Continue.