ACP > General > Server configuration > Server settings

The explain text for "Run periodic tasks from system cron" is wrong I think (see bolded portion):

When off, phpBB will arrange for periodic tasks to be run automatically. When on, phpBB will not schedule any periodic tasks by itself; a system administrator must arrange for bin/phpbbcli.php cron:run to be run by the system cron facility at regular intervals (e.g. every 5 minutes).

This is at variance with the Wiki and at least in my testing it doesn't work

https://wiki.phpbb.com/PhpBB3.1/RFC/Modular_cron#Use_system_cron

You might want to link this to the Wiki but it should probably be more like cd "/path/to/board"; ./bin/phpbbcli.php cron:run. Based on my testing so many forums are using shared hosting that multiple commands in a cron are often not allowed so perhaps it should have a caveat for dedicated or virtual hosting. My solution for shared hosts which I've tested is in the Wiki.