Seems like it is not possible to initialize the array the way you are doing it.
See this link
error C3074: an array can only be initialized with an initializer-list[
^]
In your case it should be
std::vector<std::array xmlns:std="#unknown"><byte,>> cKey { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
</std::array>
or maybe
std::array<byte,> keyTemp { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
cKey.push_back(keyTemp);