Magento – How to get country list in system.xml

It's likely not working for 1 of 2 reasons:

1. The class isn't being applied to the input elements.

If this the case, try using <frontend_class>validate-one-required</frontend_class> instead, and see whether that applies the class.

2. The validate-one-required rule isn't working as expected.

You can try using validate-one-required-by-name instead, which Magento seems to use frequently with other sets of checkboxes.

Try one (or both) combinations, and hopefully that'll do the trick.

system.xml

<field id="test" translate="label" type="label" sortOrder="20" showInDefault="1" showInWebsite="0" showInStore="0">
    <label>Hey you!</label>
</field>

config.xml

app/code/Vendor/Module/etc/config.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
        <section>
            <group>
                <test>Your values here</test>
            </group>
        </section>
    </default>
</config>

OUTPUT: