Getting "You do not have sufficient permissions to access this page." installing plugins

Websites created using my super admin account on my network are not able to install plugins. The plugins are listed, but there's no option to "add" a plugin, and when I try to install the pre-packaged "recommended" plugins for the theme I've used, I get a "You do not have sufficient permissions to access this page." error message.

I am certain this is a setting that I've missed somewhere, but I can't find it! Help!

Thank you...