cannot convert pair argument in lamda C++

I am getting error for the below code I am implementing thread safe write to files as below ::

class OpenFile
{
    string fileName;
    std::unique_lock<mutex> lck;
    static map<string, unique_ptr<mutex>> fmap;
public :
    OpenFile(string file) : fileName(file) {
        try {
            if(checkFile(file))
            {
                fmap.emplace(file, make_unique<mutex>());
            }
            else
            {
                unique_ptr<mutex> mu = std::move(fmap.find(file)->second);
                lck=std::unique_lock<mutex>(*mu);
            }
        }
        catch(string str)
        {
            cout << str << endl;
        }
    }
    void writeToFile(string str)
    {
        string fname = fileName;
        if (!checkFile(fname))
        {
            lck.lock();
            ofstream ofile(fileName, ios::out);
            ofile << "Writing to the file ";
            ofile.close();
            lck.unlock();
        }
        else
        {
            ofstream ofile(fileName, ios::out);
            ofile << "Writing to the file ";
            ofile.close();
        }
    }
    string ReadFile()
    {
        string fname = fileName;
        string line;
        if (!checkFile(fname))
        {
            lck.lock();
            ifstream ifile(fileName, ios::in);
            getline(ifile, line);
            ifile.close();
            lck.unlock();
        }
        else
        {
            ifstream ifile(fileName, ios::in);
            getline(ifile, line);
            ifile.close();
        }
     return line;
    }
    OpenFile() = delete;
    OpenFile& operator=(const OpenFile& o) = delete;
    static bool checkFile(string& fname);
};


bool OpenFile::checkFile(string& fname)
{
    if (find_if(fmap.begin(), fmap.end(), [&fname](pair<string, unique_ptr<mutex>> m)
                {
                    if (m.first == fname)
                        return true;
                    else
                        return false;
                }) == fmap.end())
    {
        return true;
    }
    else
        return false;
}

Error from VS2015 ::

Description Project Line    Suppression State
'bool OpenFile::checkFile::<lambda_2bd4a02838a970867505463f4b7b6b9e>::operator ()(const std::pair<std::string,std::unique_ptr<std::mutex,std::default_delete<_Ty>>>) const': cannot convert argument 1 from 'std::pair<const _Kty,_Ty>' to 'const std::pair<std::string,std::unique_ptr<std::mutex,std::default_delete<_Ty>>>'  BoQian  43