Remove the restrictions and/or profiles that can be blocking it. You'll find them in Settings > General > Restrictions, and Settings > General > Profiles.
One of the two you have configured to restrict Apps or Installing Apps or Downloading Apps or have set an Age Restriction. It can't be removed, it can only be restricted. If you can't find it by searching, then you have a restriction (or Profile turned on).
Check Settings>General>Restrictions, be sure Installing Apps is set to On. If it is
look on all your screens and inside all your folders. If you still can't find it go to Settings>General>Reset>Reset Home Screen Layout. This will restore the home screen to its original configuration but may move other apps around to do so.
You either moved it to a different home screen page or inside a folder, or you restricted use:
1. Settings > General > Restrictions > Installing Apps > On
2. Search home screens and folders, or
3. Settings > General > Reset > Reset Home Screen Layout