Fatal Error: incsub-support.php line 259

When installing the plugin I got this error:

Fatal error: Call to undefined function get_current_site() in /home/content/78/9725278/html/wp-content/plugins/incsub-support/incsub-support.php on line 259

I installed the plugin previously but it didn't show in the wp-admin sidebar, however it said it was installed by the WPMU DEV Plugin Dashboard.

I deleted the incsub-support plugin folder and reinstalled using manual download instead of the WPMU Dev Plugin Dashboard.

That's when I got that fatal error.