This is an operating system error and can indicate different error conditions.
These errors are not specifically documented in the ArcGIS Pro Help system.
However, in most cases, the error description provided with the error code can be used to look up additional information on the operating system.
If the error message is "Unexpected Error," consider contacting technical support for assistance. If the message attached to the 999998 error is any other message, these messages are standard operating system errors that can be looked up under websites such as Microsoft's Support and Help website (http://support.microsoft.com/search/?adv=1) for more information.