There's NULL and then there's NUL.

NULL is defined in stddef.h, is used very widely, and is a reference to a null pointer.

NUL is different - it is the first character in the standard ASCII character set, and more importantly, it is not a standard macro. You may have to define it yourself.

To define NUL, do:

#define NUL '\0'

The check should be against NULL not null.

while (fgets(string,17,f)!=NULL )

FILE and NULL are wrongly written (C is case sensitive). fopen line was missing a semicolon.

Code below compiles and runs.

#include <stdio.h>

main(void) {
FILE *file = fopen("words.txt","r"); 
if(file != NULL) {
    char line[128];
    while(fgets( line, sizeof line, file) != NULL) 
    {
    fputs ( line, stdout );
    }
    fclose ( file );
    }
}

NULL is not a built-in constant in the C or C++ languages. In fact, in C++ it's more or less obsolete, just use a plain literal 0 instead, the compiler will do the right thing depending on the context.

In newer C++ (C++11 and higher), use nullptr (as pointed out in a comment, thanks).

Otherwise, add

#include <stddef.h>

to get the NULL definition.

nullptr is C++ only; it is not needed in C because in C ((void*)0) is convertible to any other pointer type without casts.

If you really really really like to type nullptr in C, you can use

#define nullptr ((void*)0)

and it would then work mostly the same.

Notice that C has the NULL macro from <stddef.h>; it is readable too, but its expansion is implementation-defined, so it might be either ((void*)0) or 0 (or something really strange); if it expands to 0, you wouldn't get any diagnostics from

int a = NULL;

struct Statistics stats[5]; is local variable in the block used in for statement, so it isn't visible after the for loop.

Get it out of for statement.

int main(void)
{
  struct Statistics stats[5];
  //-------------------------------------------
  for(m = 0, m < num_assignments; m++)
  {
    stats[m].mean = calculate_mean(grades,n);
    stats[m].median = calculate_median(grades,n);
    stats[m].variance = calculate_mean(grades,n);
    stats[m].min = calculate_min(grades,n);
    stats[m].max = calculate_max(grades,n);
  }
  //-------------------------------------------
  display_grades_distribution(m, grades_scale,n, stats);
}

In your code

 temp = (Node*) malloc(sizeof(struct Node));

should be

 temp = malloc(sizeof(struct Node));

or, for better,

 temp = malloc(sizeof *temp);

eliminating the issue, as you need not to cast the return value of malloc() and family in C..

The error was, however, because you missed the struct keyword there.

To address the third error,

"expected declaration or statement at end of input."

well, you missed the ending } for the print() function.

FWIW, you should always check the validity of the returned pointer by malloc() and family of functions before making use of it.

Having said that,

For the first one I've searched and found out it's because of C89 and I have to declare all the variables at the top of my functions

is not true. Yes, in C89, you needed to declare all the variables at top, but that did not allow you to skip the struct keyword, anyway.

It should be valueToAdd you are using valuetoAdd .. Check case!!