Unwriteable

When you start installation, after install, and or when hosting does upgrading / changes setting php.

Hence will be read Session Save Path Unwriteable

Text can seen at step 2 from installation Joomla, if correct, finalized installation then see fix below.
Text can seen when you login in admin - System - System Info - Permissions.
mis. Session Directory / Unwriteable

To solve that is:

1. Create one parallel folders with public_html ( or domainname, or www) yours,give name session and gives permission 777
Because of many hosting which give different path, so applies one of examples following that may be precise for your hosting.

/home/username/domainnametld --> /home/username/session
/home/username/public_html --> /home/username/session
/home/username/www --> /home/username/session


2. Edits globals.php which there is in root directory where installation Joomla with FTP or FileManager and add in line #14:

ini_set('session.save_path','/home/username/session');