Your Signup Code Plugin is Blocking a Totally Separate Website

Hi, I can't log into my Brave New Web dashboard. I get an error message that reads: The Signup Code plugin is only compatible with WordPress Multisite.

I tried to install this plugin for a site hosted on a totally different web hoster (Siteground. Brave New Web is hosted on Bluehost). For some reason, this plugin is blocking access to Brave New Web on Bluehost. Can you help?