Access Denied on upload with 'Rebex tiny SFTP Server' and ssh2 node module

Please help I have always the Access denied error when i try to upload on server :

conn.on("ready", () => {

        conn.sftp((err,sftp) => {

            if (err) throw err;

            var providedFileName = util.format("%s", localFilePath);

            sftp.fastPut(providedFileName, '/',(err) => {

                console.log('Erreur', err);

            });

        });

    });

conn.connect(sftpConfig);