Structure Array Initialization
As explained in Michel Keijzers' answer, you cannot use the = operator
to assign something to a string (a character array). However, you can
initialize a string using =. There is an important distinction
between assignment, which you can do anywhere in a program, an
initialization, which can only be done at the time you define a
variable.

For example:



int i = 2;  // initialization

void setup() {
    i = 4;  // assignment
}


In your specific case, you can initialize fields as follows:

struct s_field_def fields[2] = {
    {"HALTED", FALSE},  // displays the current run state
    {"@", TRUE}         // displays indicator of system running   
};