as for php 7.0, which version of laravel ?

Maybe you should downgrade to laravel 5.5 https://laravel.com/docs/5.5/installation

I did, but still getting error Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE)

I am checking the dependencies..

Its got to be said... choose a hosting provider that uses supported versions of PHP

v5.6 has only recieved security updates for the last 15 months and will be totally end of life in December 2018.

http://php.net/supported-versions.php

@fen Did you run composer on a machine with php 7.1 and then copy the entire folder to the server?

If you do that, composer will install dependencies that depend on php 7.1. You can either run composer update on your 7.0 server to fix that, or add a version constraint to your composer.json file in your local machine to set the php dependency to 7.0.