How do I row lock?

index.js

    sequelize.transaction({
      isolationLevel: Sequelize.Transaction.ISOLATION_LEVELS.SERIALIZABLE,
      //autocommit: false, ???
    }, function (t) {
      return Config.findAll({
        where: { assigned: false },
        transaction: t,
        //lock: t.LOCK.UPDATE, ???
        //lock: t.LOCK.SHARE, ???
        //lock: t.LOCK.KEY_SHARE ???
        //lock: t.LOCK.NO_KEY_UPDATE ???
        limit: 1,
      }).then(function (configs) {
        var config = configs[0];

        config.updateAttributes({
          assigned: true,
        },{
          where: { id: config.id },
          transaction: t,
        })
        .then(function(updated_config) {
          callback(null, updated_config);
        })
      }).catch(function (err) {
        callback(err);
      });
    })

Just comment one lock line out and use findOne e.g.:

      return Config.findOne({
        where: { assigned: false },
        transaction: t,
        lock: t.LOCK.UPDATE,
      }).then(function (config) {
        if (config) {
          ...
        }

That way the selected config set will be locked. Btw.: you should insert a return in line 16, otherwise you won't be able to catch errors that occur while updating, since you do not wait for the response.
I changed your call to a findOne as you inserted a limit: 1 in your findAll... but you have to check if there's a return value if you can't be certain that this entry exists.