The "Hmm. We’re having trouble finding that site." page is considered one of the internal protected pages, so in theory toggling the appropriate setting to allow extensions on those pages should fix that issue.

Just for clarity, those "special" internal pages in the browser have privileged access to some specific user data, so the intent of having extensions off-by-default on them is to ensure extensions don't all have full access to all that data by default. However, this seems similar to extensions being disabled by default in Private Windows, which is easy to toggle, so I really wish they'd make special browser pages as easy to toggle.