You downloaded and installed Hadoop core. But you cannot find the core-site.xml file. What should you do?
Possible Solution #1
sudo find / -name core-site.xml
Possible Solution #2
Did you download the installation media with a file "-site" in its name?
Try again with a .tar.gz file without "-site" (and without "-src") in its name:
(This is the for open source version and not a specific vendor's implementation of Hadoop.)