"The directory sites/default/files is not writable" error
Install the drupal using virtual host, it will be help to manage all the permissions. Here i'm giving an example of a virtual host file with some permissions which might help you install the site. 



    &lt;Directory "/var/www/html/mysite"&gt;

    Options Indexes FollowSymLinks Includes ExecCGI
        AllowOverride All
        Require all granted
        Allow from all

    &lt;/Directory&gt;

    ServerName mysite.local
    ServerAlias www.mysite.local
    DocumentRoot /var/www/html/mysite
    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

&lt;/VirtualHost&gt;