I received a message that said, “Coming soon! We're hard at work enhancing your experience. Stay tuned—we'll be back shortly.”

When trying to access QuillBot on Google Docs from Extension > QuillBot|Paraphrasing and Summarizing Tool, you might see the following message. 

This is because you are trying to access QuillBot through a legacy add-on for Google Docs, which is not available currently. 

Workaround

As an alternative, you can install the Google Chrome extension and use their toolbar directly on Google Docs, or you can access the sidebar as well.