This will enable HPOS at BillPlz WooCommerce Plugin

index.php

<?php

/**
 * Use this code only at Development @ Local Environment. DO NOT USE AT PRODUCTION SITE
 * First, go to BillPlz for WooCommerce WordPress Plugin and then open file named `billPlzWoo.php`
 *Then, copy and please this code. Only put the code line 19 and method named `enableHposForBillPlz`
*/


 protected function __construct()
  {
    $this->define_constants();
    add_filter('option_woocommerce_billplz_settings', array(&$this, 'patch_keys_constant'), 10, 2);
    add_filter('before_woocommerce_init', [__CLASS__, 'enableHposForBillPlz']); // Add this line
  }

  public static function enableHposForBillPlz()
  {
    if (!class_exists(\Automattic\WooCommerce\Utilities\FeaturesUtil::class)) {
      FeaturesUtil::declare_compatibility('custom_order_tables', __FILE__, false);
    }

    FeaturesUtil::declare_compatibility('custom_order_tables', __FILE__, true);
  }