syntax issue on php 7.4

Option 1

Have you tried changing the function name? something similar function getClientFn($function){}? You can keep any name you want.

function getClientFn($function){
    return '\DisplaceTech\Companies\\' . $function;
}

function add_clients_metabox(){
    $screens = ['page'];
    foreach ($screens as $screen) {
        add_meta_box(
            'clients',              // Unique ID
            __('Clients'),          // Box title
            getClientFn('clients_box_html'), // Content callback, must be of type callable
            $screen,                // Post type
            'advanced',             // Context
            'high'                  // Priority
        );
    }
}
add_action('add_meta_boxes', 'add_clients_metabox');

Option 2: If you really need to keep ‘fn’ method, then give a try with below script which works in php 7.4.

$myfunction = fn($function) => '\DisplaceTech\Companies\\' . $function;

function add_clients_metabox(){
    $screens = ['page'];
    foreach ($screens as $screen) {
        add_meta_box(
            'clients',              // Unique ID
            __('Clients'),          // Box title
            $myfunction('clients_box_html'), // Content callback, must be of type callable
            $screen,                // Post type
            'advanced',             // Context
            'high'                  // Priority
        );
    } } add_action('add_meta_boxes', 'add_clients_metabox');