You have to set up a new account with the codes it won't work in your current xbox accounts. Also the codes are being sent out in batches they're behind as they didn't expect the amount of gaming orders.
Also make sure you download the xbox app from Samsungs store and not Google play as the code won't work either
currently speaking to samsung now, they keep passing the blame, but just so everyone is aware SAMSUNG have breached their own T&Cs which mean you can be compensated.
the T&Cs state that if you redeem your evoucher before 15th September youll recieve the code on the 15th, if you redeem on or after 15th you will receive the code within 24 hours.
nothing in the T and Cs say that from the 15th the codes will be sent out in batches.
that means samsung have breached the terms and conditions, Trading Standards have also backed me and said that they have also and especially as it was a promotion to bring in specific buyers (people who enjoy xbox amd gaming).