[Ultimate Branding] not working on WordPress "monosite"

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The plugin page says "Completely white label your entire WordPress, Multisite or BuddyPress install, using this one powerful plugin."

Great. I install it and I get a "[Ultimate Branding] Plugin only works in Multisite."

Why?

Is clearly writte that it work with WordPress "monosite" and even providing that it doesn't: why wouldn't? I don't see any reason not to make this plugin working with a normal WP installation.

And by the way, this forum doesn't work: I have to select a plugin, but I get only a list of 3 plugin.