Error Valu_type does not name a function C++

#include <iostream>
#include <cassert>
#include <cstdlib>
#include "sequence.h"

using namespace std;

sequence::sequence ( )
{
    current_index = 0;
    used = 0;
}

sequence::size_type sequence::size( ) const
{
    return used;
}

void sequence::start ( )
{
    current_index = 0;
}

sequence::value_type sequence::current( ) const
{
    return data[current_index];
}

void sequence::advance ( )
{
    assert (is_item());
    current_index++;
}

bool sequence::is_item( ) const
{
    return current_index < used;
}

void sequence::insert (const value_type& entry)
{
    assert( size( ) < CAPACITY);
    for (int i = used; i > current_index; i--)
    {
        data[i] = data[i-1];
        data[current_index] = entry;
        used++;
    }
}

void sequence::attach (const value_type& entry)
{
    assert( size( ) < CAPACITY);
    for (int i = used; i > current_index; i--)
    {
        data[i] = data[i+1];
        data[current_index] = entry;
        used++;
    }
}

void sequence::remove_current( )
{
    assert (is_item());
    for (int i = current_index+1; i < used-1; i++)
    {
        data[i] = data[i+1];
        used--;
    }
}

ostream& operator <<(ostream& outs, const sequence& source)
{
    outs << source.current() << " ";
    return outs;
}

This is specifically where i am having problems and the only error code i am getting is the one listed in the title.

value_type operator[](size_type index)const
{
          if( index > SIZE )
          {
              cout << "Index out of bounds" <<endl;

              return sequence[0];
          }
          return sequence[i];
}