CK999, the message class and number is standard, but in standard SAP, it doesn't include any text, even this message is not used in any standard program. The message has been customized by your organization.
Go to SE91, enter the message class CK and press display, select the message number (999) and click Where used list, then select program and press enter. System will give you the program name where the message is used. Then consult with your ABAPer to check the root causes of the error message.
