I have tried installing the plg_system_t3.v2.7.0.zip by uploading a package and by using the upload by folder.
By package error: > Error
There was an error uploading this file to the server.
Maximum PHP file upload size is too small: This is set in php.ini in both upload_max_filesize and post_max_size settings of your PHP settings (located in php.ini and/or .htaccess file).
Unable to find install package
By folder: > Warning
JInstaller: :Install: Can’t find XML setup file.
The Joomla package cannot be installed through the Extension Manager. Please use the Joomla! Update component to update Joomla.
Path does not have a valid package.
As you can see in the error: http://prntscr.com/hamcag
The size for uploading is too low, Kindly increase the upload file size so the plugin will install.
At this time it’s not uploaded because of the size limit on your server.