Not possible to complete order with free products

So the issue here is that the order needs to be completed from My Orders page, thus it’s not a bug in WooCommerce but a customization. Here is how to solve it.

The code that causes the problem is located here:

# includes/wc-class-order.php

        /**
	 * Checks if an order needs payment, based on status and order total.
	 *
	 * @return bool
	 */
	public function needs_payment() {
		$valid_order_statuses = apply_filters( 'woocommerce_valid_order_statuses_for_payment', array( 'pending', 'failed' ), $this );
		return apply_filters( 'woocommerce_order_needs_payment', ( $this->has_status( $valid_order_statuses ) && $this->get_total() > 0 ), $this, $valid_order_statuses );
	}

More specifically this causes the problem $this->get_total() > 0

So to override this filter, add this to your functions.php

//Allow pay for order when the total price is $0.00 in frontend
function check_order_payment($th, $order, $valid_order_statuses) {
  return $order->has_status( $valid_order_statuses );
}
remove_filter('woocommerce_order_needs_payment', WC_Order);
add_filter( 'woocommerce_order_needs_payment', 'check_order_payment', 10, 3 );