Since you’re using the file driver, you need to make sure that Apache has access to create and write to the session files. Run these commands to give Apache permission to do so:

sudo chown www-data:www-data /path/to/laravel/storage
sudo chmod og+rw /path/to/laravel/storage

More specifically, session files are stored in storage/framework/sessions, but Laravel needs write access to all of storage anyway.