It can be that Droppy is unable to install the plugin you're trying to install, this can be caused my multiple things.
- Make sure the the PHP Zip module has been installed and enabled, the installation needs to unzip a zip file and without this extension it won't be able to proceed.
- You can try to set the directory permissions of the plugins/ directory to 777, then install the plugin by using the plugin installer in your admin panel. Do not forget to change the permissions back to 755 when you've successfully installed the plugin
- Make sure your PHP settings are configured properly, we suggest to give a look to this article for more information about this.