Resolve the Error
Check to see if any filters are defined on the Object and make sure any modifications are adhering to the Filter Criteria. You can check if there are any filters on the Object by following these steps:
1. Go to Setup | Customize | <Object>| Fields.
2. Scroll through your Standard and Custom Fields until you find a Lookup Field.
3. Click on the Field Label and review the detail page to see if any filters exist.
4. Review the Filter Criteria to be sure your modifications adhere to it.