What does "request for member '*******' in something not a structure or union" mean?

It also happens if you're trying to access an instance when you have a pointer, and vice versa:

struct foo
{
  int x, y, z;
};

struct foo a, *b = &a;

b.x = 12;  /* This will generate the error, should be b->x or (*b).x */

As pointed out in a comment, this can be made excruciating if someone goes and typedefs a pointer, i.e. includes the * in a typedef, like so:

typedef struct foo* Foo;

Because then you get code that looks like it's dealing with instances, when in fact it's dealing with pointers:

Foo a_foo = get_a_brand_new_foo();
a_foo->field = FANTASTIC_VALUE;

Note how the above looks as if it should be written a_foo.field, but that would fail since Foo is a pointer to struct. I strongly recommend against typedef:ed pointers in C. Pointers are important, don't hide your asterisks. Let them shine.

You are trying to access a member of a structure, but in something that is not a structure. For example:

struct {
    int a;
    int b;
} foo;
int fum;
fum.d = 5;

It may also happen in the following case:

eg. if we consider the push function of a stack:

typedef struct stack
{
    int a[20];
    int head;
}stack;

void push(stack **s)
{
    int data;
    printf("Enter data:");
    scanf("%d",&(*s->a[++*s->head])); /* this is where the error is*/
}

main()
{
    stack *s;
    s=(stack *)calloc(1,sizeof(stack));
    s->head=-1;
    push(&s);
    return 0;
}

The error is in the push function and in the commented line. The pointer s has to be included within the parentheses. The correct code:

scanf("%d",&( (*s)->a[++(*s)->head]));