Make a Select Form Field Required

When adding a form rule for select (or multiple select) fields, these need to be applied on the client side (not server side).

Here’s an example of how to add a rule for a select field:

$mform->addRule(
    'selectfield', 
    get_string("selectfieldrequiredmessage", "local_yourplugin"), 
    'required',
    '',
    'client'
);   

The key is the 5th parameter above which is set to client instead of the default of server.

For more help, see the Moodle Forms Library Documentation