The GD library is not installed on your server. This library is required to produce the sparklines. Please ask your system administrator or your web host to install
php-gd for you. For more information, check the Matomo Diagnostics Check.
On linux, you can install php-gd by executing
apt-get install php-gd on debian type systems, then restart your webserver.
If you are using Apache webserver and a module called
mod_security, you may need to disable this mod_security module as it may sometimes cause Sparklines graphs to not display.