People won't use concepts unless they are using C++20 (not many use C++20).

Anyway I fixed your code for you without using concepts. You are missing the typename keyword.

template<typename T1>
struct {
  vector<T1> values;
  
  template<typename T2, typename = enable_if<is_convertible<T1, T2>::value>::type>
  void add(T2 v) {
    values.push_back(static_cast<T1>(v));
  }
};

Also your static cast type was wrong.