MacOS Sierra is downloaded but can’t install

Start up with safe mode and then try to download and install. Here is how to enter into Safe Mode.

I have same problem and succeed to upgrade to Sierra in Safe Mode.

Good luck!

Whether or not you've downloaded it in the past, you can now download OS X El Capitan 10.11.6 (6.21 GB) via this link which will open first in the Browser, then in the App Store.

OS X El Capitan

This version of OS X El Capitan is for users running OS X Snow Leopard who would like to upgrade to macOS Sierra. If you are running Snow Leopard and would like to upgrade to Sierra, you need to install El Capitan first. This version of El Capitan can also be installed on Mac computers that are not comparable with Sierra.

Note: If doing this to downgrade from macOS Sierra.

In order to use the Install OS X El Capitan.app from macOS Sierra you need to create a USB Installer and boot the Mac with it. You'll also need to erase the Macintosh HD and perform a clean install. Make sure you have proper User Data Backups from which to restore your Data before erasing the Macintosh HD.

Have a look at Create a bootable installer for OS X. Excerpt show below:

Use the 'createinstallmedia' command in Terminal

1. Download the OS X installer from the Mac App Store. Quit the installer if it opens automatically after downloading. The installer will be in your Applications folder.
2. Mount your USB flash drive or other volume. You could also use a secondary internal partition.
3. Open the Terminal app, which is in the Utilities folder of your Applications folder.
4. Use the createinstallmedia command in Terminal to create the bootable installer. Examples of this command are in the next section. For detailed usage instructions, make sure that the appropriate Install OS X app is in your Applications folder, then enter one of the following paths in Terminal:

Path for El Capitan:

/Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia

Examples

This is the basic syntax of the command. Replace volumepath with the path to your USB flash drive or other volume, and replace installerpath with the path to the Install OS X app.

createinstallmedia --volume volumepath --applicationpath installerpath

The following examples assume that the OS X installer is in your Applications folder and the name of your USB flash drive or other volume is MyVolume:

Example for El Capitan:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app