One possibility is that your WordPress site is hosted on a low-quality or shared hosting plan. In this case, your host may not allow you to install plugins because they can slow down the site or create security vulnerabilities. Another possibility is that you don't have the necessary permissions to install plugins. If you're using a self-hosted WordPress site, you'll need to have admin privileges in order to install plugins. And if you're using WordPress.com, you'll need to upgrade your account to a paid plan in order to access the plugin installer.