Restrict WooCommerce POS Pro users to one (or more) stores

functions.php

<?php 

// the code below goes in your theme's functions.php file

function my_custom_pos_template_redirect() {
  
  // a manual map of Store IDs => array of User IDs
  $restrict = array(
    1524 => array( 52, 863 ),  // ie: Store ID 1524 restricted to User IDs 52 and 863
    6252 => array( 863 )       // ie: Store ID 6252 restricted to User ID 863
  );
  
  // get current User ID & logged in Store ID
  $user_id = get_current_user_id();
  $store_id = get_user_option('woocommerce_pos_store');
  
  // check User ID array exists and User ID in array, if not, display error
  if( ! isset( $restrict[$store_id] ) || ! in_array( $user_id, $restrict[$store_id] ) ) {
    wp_die( 'You do not have sufficient permissions to access this store.' );
  }
  
}

// woocommerce_pos_template_redirect runs just before the POS template is rendered
add_action( 'woocommerce_pos_template_redirect', 'my_custom_pos_template_redirect', 20 );