Error with << operator, "no operator "<> << void"

My code is for a class which already has an oprator<< function

friend ostream& operator<<(ostream &os, const Alist& other);

ostream& operator<<(ostream &os, const Alist& other)
{
  for (int i = 0; i <= other.val; i++)
   os << other.a[i] << ", ";
  return os;  
}

it doesn't satisfy the parameters for this statement in my main function

cout << "Double remove: " << a1.remove(n) << " " << a1.remove(n) << endl;

and I receive this error

no operator "<<" matches these operands -- operand types are: std::basic_ostream<char, std::char_traits<char>> << void

I'm not sure how to go about writing the function for another << operator but so far I have what's shown below as that seems to meet the needs.

friend ostream& operator<<(ostream &os, const unsigned char *s);

is this correct? and how would I write the function for this? Below is the remove function.

bool Alist::remove(int n)
{
  for (int i = 0; i < val; i++)
   if(a[i] = n)
    {
      for ( int j = i; j < (val - 1); j++)
        a[j] = a[j+1];
    }
  for (int i = 0; i < val; i++)
   if(a[i] != n)
    return true;
  else
    return false;
}