"Error 999999: Error executing function" - a generic error in Esri ArcGIS products without a specific error ID. Basically it means "uh-oh something bad happened".
For some error cases, additional messages may accompany the 999999 error. As well, checking the log file for accompanying internal system errors may reveal information about the nature of the error; otherwise, consider contacting technical support for assistance.
The reference page is for ArcGIS Pro, but the definition is the same for ArcGIS Desktop going back to at least 10.0.