This error is an error is an error within Crystal Reports.  

It is often caused by copying and pasting fields into different portions of the Crystal Report design.  This can essentially "break the report" and require the report be built again.  

This link provides some additional information on this topic.

Note: We provide links to third-party websites in an effort to help you resolve your issue.  We are not responsible for the information on third-party websites and we cannot assist with implementing resolutions from these websites.

Crystal Reports is not a Blackbaud program, we're not fully equipped to troubleshoot errors occurring within it   If you find that you need further assistance with your Crystal Report, please contact a Crystal Report expert to assist you further. 

The Blackbaud Community may have other users who will be able to assist you; you may want to reach out to others there to see if they've encountered similar issues or have suggestions about how to find a Crystal Reports Expert. Here's a link to that page: