This can happen if you try to add a premium to a donation form that does not have a premium selector included on the form. This can happen if you try and add a premium long after creating the donation form. To fix this problem:
  1. Edit the donation form that appears in the error message
  2. Go to #3 "Design Donor Screens"
  3. Click edit next to the donation form
  4. Find "premium selector" on the lefthand side scrollbox and add it to the form