Ok I find the solution : The app needed the licence key inside the config file for the verification of the purchases (otherwise it is “verificationFailed”).
The licence key can be founded inside the google game account on the section “Monetize > Configuration of monetization > Licence”
application =
{
license =
{
google =
{
key = "MIIBIjANBgkqhkiG9 (...) 5VlN1OEnZOeHMQIDAQAB",
},
},
}
You can see the part of the code at the end of that page :