function llms_check_access_plan_purchasable( $plan, $user_id = null ) {

	$can_purchase = true;

	if ( ! $plan || ! is_a( $plan, 'LLMS_Access_Plan' ) ) {
		$can_purchase = new WP_Error( 'invalid-plan-id', __( 'Invalid Access Plan ID.', 'lifterlms' ) );
	} elseif ( ! $plan->is_available_to_user( $user_id ) ) {
		$can_purchase = new WP_Error( 'plan-not-available', __( 'This access plan is not available.', 'lifterlms' ) );
	} else {
		$product = $plan->get_product();
		if ( ! $product || $product->has_restrictions() ) {
			$can_purchase = new WP_Error( 'product-not-purchasable', __( 'This product is not available for purchase.', 'lifterlms' ) );
		}
	}

	/**
	 * Filters whether an access plan can be purchased during checkout.
	 *
	 * @since 10.0.8
	 *
	 * @param true|WP_Error         $can_purchase Whether the plan can be purchased. `true` if it can, a `WP_Error` otherwise.
	 * @param LLMS_Access_Plan|null $plan         The access plan being purchased.
	 * @param int|null              $user_id      WP_User ID of the purchasing user, or `null` for the current user.
	 */
	return apply_filters( 'llms_check_access_plan_purchasable', $can_purchase, $plan, $user_id );
}