There was an unexpected error during the Add Field process. This can happen when a file is in use by another program, the file is a read-only data source (such as a CAD file), the input has read-only permissions, or the disk is full.
Check whether the table is locked by another user or is read-only. You can try using different output files. If that doesn't work, try closing the application and running the tool again.
If the input was created using GeoAnalytics tools and saved to the ArcGIS Data Store of type spatiotemporal, adding fields is not supported.