Q:

Grub installation failed during Ubuntu installation

I want to use Ubuntu 12.04 Long Term Support  but during installation it showed me a message

Grub can't be installed.

how can I install manually. 

A:

Try with Boot Repair.
1 - Live CD
First, you'll need to boot from a Ubuntu Live CD.
2 - Repair the bootloader
To restore the Grub bootloader, follow these steps.
Open a terminal from the Live CD, and run the following commands:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair
boot-repair

After a few seconds, Boot Repair's main window will open.
Click on Recommended repair.
You're done!
Reboot to test everything out.
If everything went well, you should be up and running now.

Glossary

Bootloader: a piece of code that runs before any operating system is running. A bootloader is used to boot other operating
  systems. Each operating system usually has its own bootloader.
GRUB (Grand Unified Bootloader): a bootloader package developed to support multiple operating systems and allow the user to select among them during boot-up. It is Ubuntu's default bootloader.
Command, command line: the traditional Unix environment , where you type commands to tell the computer what to do.
Terminal: an application that allows you to access the command line. Open it by hitting its keyboard shortcut, Ctrl Alt T

