The Value of Goods and/or Services must be set.

This can be resolved by doing the following:

1. Edit the associated Donation Form.
2. Click step 2. Configure Donation Levels.
3. Click Edit next to the level you'd like to update.
4. In step 3. Value of Goods and/or Services, enter the full amount of the donation.  (ie put "25" for the $25 Donation Level)
5. Click Save.