This error will occur if the original, system-generated Donor Proxy for the Organization was deleted or moved from the Organization account. Once the Donor Proxy is deleted or removed from the Organization record, it cannot be used and cannot be replaced. You can confirm this by viewing both the Organization record and the Donor Proxy record. The "Created" fields should match if the current Donor Proxy is the original system-generated Donor Proxy. If the fields do not match, this Donor Proxy record is not the original and cannot be used to assign gifts to the Organization account.

To correct this, you will need:
  1. Recreate the Organization account record to generate a new original, system-generated Donor Proxy
  2. Reassign gifts to this new Organization and Donor Proxy
  3. Remove the old Organization account record