Assigning value inside structure array outside setup and loop functions
You can't access array contents in the global scope like that.  You have to assign the entire block of data in one go:

struct trap_game_detail {
    byte flush_quantity;
    byte flush_interval;
    byte random_min;
    byte random_max;
    byte random_quantity;
};

struct trap_game_details trap_game_details[2] = {
    { 10, 5, 1, 100, 4 },
    { 8, 23, 5, 95, 8 }
};


Items in the global scope aren't executed - only things in functions are executed.  Accessing individual parts of an array or struct like you are trying to is a programatic operation, so can't be executed since it's not in a function.