It would really help to show a complete small example and the error message you are getting.
This code works for me
module top ;
typedef struct {
int mode;
int pair_num;
} my_cal_s;
my_cal_s my_cal_arr[];
initial begin
my_cal_arr = '{'{0,2}, '{1,4}, '{0,3}, '{2, 8}};
end
endmodule