Hello Philippe,
Thank you for checking and confirming. Regarding your new question: The reason because the product option value is out of stock (quantity = 0) so you can not checkout. There are 2 ways to resolve it:
1. Edit product, under Inventory tab, at Stock Checkout choose Yes. This allow customer to checkout product althought it is out of stock.
2. Edit product, click on Option tab to enter the quantity for each option value if you want to manage inventory of the product.
I hope this is useful.
Sincerely, Giang
