The plugin is under active development and will need to be installed manually, here's how: https://www.godaddy.com/help/install-a-wordpress-plugin-from-a-zip-file-40849

Some screenshots of the button added on a placeholder article: https://github.com/stefanbohacek/fediverse-share-button-wordpress/blob/main/assets/screenshot-1.png and the settings page: https://github.com/stefanbohacek/fediverse-share-button-wordpress/blob/main/assets/screenshot-2.png