Array Maximum Constant Expression Required Error

This error shows up when i try to find max array from these variables: Constant Expression Required. i've had a go, any ideas?

 double num_array[3];
       num_array[0]= RFTPD8;
       num_array[1]= RFTPD7;
       num_array[2]= RFS3;
    
        double num_array[3]= {RFTPD8,RFTPD7,RFS3};
       int maxValueIdx=ArrayMaximum(num_array,WHOLE_ARRAY,0);