Hyperlinks not working in index.html

It is my first step learning HTML so, I have an issue with hyperlinks on index.html they are not redirecting.

after spending time I found the issue.
I decided to note

The reason was letter case of the filename extensions. The scripts were with upper-case (CAPITALS) (.HTML) extensions which my other sub-pages which I wanted to redirect were created with lower-case (.html) extensions. This mismatching was the reason.
As I edited them back to make identity, it is working now


in developers’ language, it means the extension is case sensitive.