The way I launch it is to run it directly from a command line on the server.
After I navigate to the correct folder I type
php enrol_database_sync.php
Of course this won't help if you don't have access to the server's command line. If php isn't set up in the path you may need to add the path to the php e.g. /bin/usr/php
The following might work
http://<your website/moodle/enrol/database/enrol_database_sync.php
when you are logged in as admin.
Enrolment
launching the enrol_database_sync.php script
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