Here is how to check if your discount is configured for Stores or Products:
  1. Navigate to Fundraising > eCommerce
  2. Select the Discounts tab
  3. Locate the desired discount; click Edit
  4. Click 5. Application List
  5. Click Configure in the Products row
  6. Click Remove All; click Save
  7. Click Configure in the Stores row
  8. Add the desired store(s) to the column on the right; click Save
You will now be able to return to Step 2 and configure the discount's tiers without receiving this error.