Passing string to struct in C

There is no space to allocate your string in line, because it is just a no initialized pointer, so you will have to reserve some memory for it. Furthermore, your function is allocating memory which is not being deallocated (and regarding helper, you wouldn't need two different bunchs of memory for the same string).

#include<stdlib.h>
#include<stdio.h>

const uint8_t MAX_NAME_SIZE = 20;

typedef struct label{
    char *line;
}label;

void save_line(label text){
    printf("Write your name (max len %u chrs):\n",(MAX_NAME_SIZE-1));
    scanf("%s", text.line);
}

void main(){
    label text;
    text.line = malloc(MAX_NAME_SIZE);
    save_line(text);
    printf("%s", text.line);
    free(text.line);
}

I prefer to allocate and deallocate memory in the same scope, to avoid memory leaks (inside the same function, inside the main, etc), so in order to keep your structure, for the example the memory is being managed in the main. You should think about your design, and analyse the scope that label text; should have.

Your code is wrong at many levels:

You probably want something like this:

#include <stdio.h>
#include <stdlib.h>

typedef struct label {
  char *line;
}label;

void save_line(label *text) {   // use label *ttext instead of label text
  printf("Write your name\n");
  text->line = malloc(30 * sizeof *text->line);   // no need for strcpy here anyway 
  scanf("%s", text->line);
}

int main() {               // main should return int *
  label text;
  save_line(&text);        // pass the pointer to text, not just text
  printf("%s", text.line);
  free(text.line);         // free allocated memory
}

There is still room for improvement though, e.g. scanf("%s"), ... is dangerous because if the user types too many characters, you'll get a buffer overflow.

*read this: What should main() return in C and C++?

strcpy only copies the thing pointed to by the source to the destination. It does not allocate space in the destination for the stuff being copied.

If your platform has strdup you can use that instead:

text.line = strdup(helper);

If not, allocate some space in text.line before you strcpy

text.line = malloc(strlen(helper) + 1); // +1 for the null byte at the end
 strcpy(text.line, helper);