Log in as admin Go to Site administration > Users > Accounts > Cohorts Create 2 empty cohorts, no need to attribute users to it. Go to a course and then Participants. Click Enrol users button. Select more than one user to be enrolled (at least 2). Click Enrol selected users and cohorts button. Look apache error logs usually {{ tail -100 /var/log/apache2/error.log}}. Make sure you don't see the error mentioned on the description. Now, click on Enrol users button again. This time, select just a user and select multiple cohorts (at least 2). Enrol the user and have a look on the log again. You should not see the same error, but this time for cohort list: [Mon Oct 23 12:52:18.805607 2017] [:error] [pid 12679] [client 127.0.0.1:55994] Debugging: Invalid array parameter detected in required_param(): cohortlist in \n* line 657 of /lib/moodlelib.php: call to debugging()\n* line 78 of /enrol/manual/ajax.php: call to optional_param()\n, referer: http://simey.per.in.moodle.com/stable_master/user/index.php?id=28