error: passing 'const …' as 'this' argument of '…' discards qualifiers

The error message tells you that you that you are casting of const from your object in operator< function. You should add const to all member functions that don't modify member.

bool operator<(const stockType& stock) const
//                                     ^^^^^
{
  return (symbols < stock.symbols)
}

The reason why compiler complains about operator< is because std::sort uses operator< to compare the elements.

Also you have another syntax error in insert function.

Update:

void insert(const& stockType item);

to:

void insert(const stockType& item);
//                         ^^