The data directory exists, but is not writable by the web server.
Please make sure the web server has permission to write to the
data directory.
Please make sure the database you have chosen exists and is empty.
The username provided must have access to create tables within the database.
SQLite with default settings is fine for tens of users with thousands of images. For thousands of users or millions of images, postgres is recommended.
Drivers can generally be downloaded with your OS package manager; for Debian / Ubuntu you want php-pgsql, php-mysql, or php-sqlite3.