One or more .las files are no longer present in their referenced path, or the .las file is invalid. An invalid .las file is typically encountered when there are incorrect entries in the LAS header, variable length records, or extended variable length records.
Review the LAS dataset's properties in the Catalog view to determine the presence of missing files. If files are missing, replace them with valid path references. If all files are present, the LAS dataset is referencing one or more invalid .las files. Review the variable length records in the .las file's properties dialog box and determine if any records that do not convey legible information are present. Any files with invalid variable length records should be recreated by using the Convert LAS tool with the Remove Variable Length Records option.