{"busy_timeout","test"}
is not the value of a map, but a pair. You need {{"busy_timeout","test"}}
.
Your code should look like this:
static std::map<std::string, std::map<std::string, std::string>> _ScalingMapVolume = {
{"AA", {{"busy_timeout", "test"}}},
{"BB", {{"cache_size", "10000"}}}
};