A failed NX purchase showing Error code 000-400-0006 means that the account has reached its NX Credit purchasing limit, and further Nx Credit cannot be purchased until the limit has been reset, usually within 1-2 weeks.
Please see below for an example of the aforementioned error message:
NX Prepaid can still be purchased on an account, has less restrictions on what you can purchase in-game, can be used to gift friends, and does not have any purchasing limits.