Like Wout says the error means that you still have the page selected within one of those things. But the pages has been deleted so it can not be used. Just delete the action that make use of show the page and the error should be gone.

This document shows some things where it could be: https://docs.mendix.com/refguide/web-modeler/consistency-errors-navigation-wm

Hope it will help you solve the problem!

It looks like a button, microflow or navigation is still referring to your deleted page. The error list usually indicates which element refers to this.
Can you upload a screenshot of the error list if you can not find the reference?

Thanks everyone, I had accidently deleted a page. Figured it out with your help!