Boost bimap can't convert const CompatibleKey to Key&

It is one of the first times I am using boost and I am getting an error saying

BaseKey boost::bimaps::container_adaptor::detail::key_to_base_identity<BaseKey,KeyType>::operator ()(Key &) const': cannot convert argument 1 from 'const CompatibleKey' to 'Key &

and

boost::multi_index::detail::ordered_index_impl<KeyFromValue,Compare,SuperMeta,TagList,Category, AugmentPolicy>::find': no matching overloaded function found

I know most of the STL errors or at least where could they come from, but I am not experienced enough with boost to know what could be going on here. The code I have is the following, it is used to convert the values from an enum to strings and vice versa.

file.h

namespace FOO_NS::BAR_NS
{
class FooClass
{
  public:
   enum class Enum
    {
      Enum1, Enum2, Enum3, Enum4
    };

    ...
};

namespace
  {
    using results_bimap = boost::bimap<FooClass::Enum, std::string>;
    using position = results_bimap::value_type;
    const auto EnumsAsStrings = []() {
      results_bimap result;
      result.insert(position(FooClass::Enum::Enum1, "Enum1"));
      result.insert(position(FooClass::Enum::Enum2, "Enum2"));
      result.insert(position(FooClass::Enum::Enum3, "Enum3"));
      result.insert(position(FooClass::Enum::Enum4, "Enum4"));
      return result;
    };
  }  // namespace
}//namespace FOO_NS::BAR_NS

file.cpp

using namespace FOO_NS::BAR_NS;
void doSmth()
{
 ...
 std::string enumString = EnumsAsStrings().left.at(FooClass::Enum::Enum1); // Expected string "Enum1"
}

Do you see any misconception or misusage I have in this code so that this mentioned error happens?

Okay, so at the end it wasn't anything related to this code (directly). I was calling the lambda like this EnumsAsStrings().left.at(FooClass::Enum1) and it couldn't implicitly convert from FooClass to FooClass::Enum and that was creating the errors. Thank you to everyone who tried to answer my question!