Android Device doesn't have a Clock app So, here is my problem: One time, I am setting an alarm, but I noticed that my phone doesn't have a stocked Clock app. I also go to /system/app/ClockPackage_ESS.apk using ES File Explorer. I installed that .apk, but when I'm trying to install it, it says "File not installed."

Here is the screenshot (click to enlarge) of the App Drawer:



Also, here's the device information:


  Device Information:
  Samsung Galaxy Core Prime
  Model number: SM-G360HU
  Android version: 4.4.4 (Kitkat)
  Baseband version: G360HUXXU0ANL6
  Kernel version:
     3.10.17-388893
     dpi@SWDD6214 #2
     Thu Feb 5 16:44:31 KST 2015
  Build number: KTU84P.G360HUXXU0AOB1


Does someone know how to fix this problem?
Your phone does have clock if there's that app inside system/app as you told. Its not necessary that it must be shown on launcher, though normally clock apps are shown. I suggest you give a try to some third party launcher on store and see if that does some trick. On contrary, you should be able to access clock by tapping the clock in notification panel.

Also, if there's some app inside system/app, its already installed. And apps inside this folder are called system apps. You cant install them like normal apps by simply tapping and pressing on install (or rather they dont need to be installed like normal). You install them by simply pushing their apk file to system/app or system/priv-app depending on type of app and then setting their permission to 755 or rw-r-r with some file manager. So that "install failed" is justified. In other words, app is already installed.