Errors on installing Support Plugin

Hello,

I'm getting errors on installing the Support Plugin:
Fatal error: Call to undefined function get_current_site() in /home/thecity/public_html/wp-content/plugins/incsub-support/incsub-support.php on line 259
After it the plugin appears on the plugins list but trying to activate it, I'm getting the same error.
On the first time I deleted it, but when I tried to install again, I got the same errors.

Any idea?

Thank you