During Month End validation checks, you may note the above error is displayed following the Month End validation checks.
This is generated because an owner record is inactive, with charges are recorded against a horse that is attributed to the person in your database.
You may need to make the owner active again to be able to resolve this validation issue.
To set an owner record to active;
Click People & Firms
Change View by to Non-Current People
Search for the Owner name
Click the box beside Current person / organisation
For more information on the Month End validation errors, consult the Ardex User Guide Month End Statements Guide - Validation Checks (pg. 5)