My problem resolved,
I referred to this link was updating successfully.
link - https://moodle.org/mod/forum/discuss.php?d=218731
Updated backup/moodle2/backup_stepslib.php
class backup_course_structure_step extends backup_structure_step {
protected function define_structure() {
global $DB;
// Define each element separated
$course = new backup_nested_element('course', array('id', 'contextid'), array(
'shortname', 'fullname', 'idnumber',
'summary', 'summaryformat', 'format', 'showgrades',
'newsitems', 'startdate', 'enddate',
'marker', 'maxbytes', 'legacyfiles', 'showreports',
'visible', 'groupmode', 'groupmodeforce',
'defaultgroupingid', 'lang', 'theme',
'timecreated', 'timemodified',
'requested',
'enablecompletion', 'completionstartonenrol', 'completionnotify', 'coursegroup')); // coursegoup is custom field
Hope this will be help for anyone facing same problem.