The error message, "Can't reach this page" is returned when launching a geoprocessing tool help page if certain Windows updates are installed.

Description

When launching geoprocessing tool help or help from other points in the ArcMap 10.x user interface, the following error messages may be returned:

"Can't reach this page"

or

“This page can’t be displayed”

Cause

This is due to a recent Windows software update.

Update (12/3/2018): 
Microsoft has confirmed that the issue was introduced with security update KB 4458469.  Esri Software Development is in close communication with Microsoft while they work to resolve this issue.

Update (11/28/2018):
Microsoft acknowledges a previous Windows patch is causing this unintended issue with .chm files, and they are determining the best way to address it.

Workaround

If either of these error messages is returned in the help, it is still possible to navigate to the desired help topics through the Contents tab, or find help topics through the Search tab.

Esri Software Development is in close communication with Microsoft while they work to resolve this issue. This article is updated as more information becomes available.

Update (1/25/2019):
Microsoft released a fix to address this issue. Microsoft's patch includes a fix as listed below:
"Addresses an issue that may prevent some applications from displaying the Help (F1) window correctly"

Below are links to some of the patches:

• https://support.microsoft.com/en-us/help/4480955/windows-7-update-kb4480955
• https://support.microsoft.com/en-us/help/4480976/windows-10-update-kb4480976

Note: 
You must download a patch from Microsoft that is specific to your OS version and build.

Steps to Reproduce