macOS siera reinstall fails after download

What method are you using to reinstall Sierra.

Are you trying using the Apple Recovery Servers

or have downloaded the InstallOS.dmg from here,

How to upgrade to macOS Sierra - Apple Support

and then created the Install macOS.app.

One common cause iis failing Hard Drive...

Repair a storage device in Disk Utility on Mac

Disk Utility can fix certain disk problems—for example, multiple apps quit unexpectedly, a file is corrupted, an external device doesn’t work properly, or your computer won’t start up. Disk Utility can’t detect or repair all problems that a disk may have.

If you run First Aid on a disk, Disk Utility checks the partition maps on the disk and performs some additional checks, and then checks each volume. If you run First Aid on a volume, Disk Utility verifies all the contents of that volume only.

1. In the Disk Utility app  on your Mac, choose View > Show All Devices.
2. Note: If you’re checking your startup disk or startup volume, restart your computer in macOS Recovery, select Disk Utility in the macOS Utilities window, then click Continue. If you check your startup volume (Macintosh HD), make sure you also check your data volume (Macintosh HD - Data).
3. 
   
4. In the sidebar, select a disk or volume, then click the First Aid button .
5. If Disk Utility tells you the disk is about to fail, back up your data and replace the disk—you can’t repair it. Otherwise, continue to the next step.
6. Click Run, then click Continue.
7. If Disk Utility reports that the disk appears to be OK or has been repaired, you’re done. You can click Show Details to see more information about the repairs. Otherwise, you may need to do one of the following.

• • If Disk Utility reports “overlapped extent allocation” errors, two or more files occupy the same space on your disk, and at least one of them is likely to be corrupted. You need to check each file in the list of affected files. Most of the files in the list have aliases in a DamagedFiles folder at the top level of your disk.
    • If you can replace a file or re-create it, delete it.
    • If it contains information you need, open it and examine its data to make sure it hasn’t been corrupted.
  • If Disk Utility can’t repair your disk, or you receive a report that the First Aid process failed, try to repair the disk or partition again. If that doesn’t work, back up as much of your data as possible, reformat the disk, reinstall macOS, then restore your backed-up data.

If your Mac has a Fusion Drive and you see a flashing question mark or alert, see the troubleshooting section of the Apple Support article About Fusion Drive, a storage option for some Mac computers.

If you continue to have problems with your disk or it can’t be repaired, it may be physically damaged and need to be replaced. For information about servicing your Mac, see Find out how to service or repair your Mac.

So you are trying to use the Recovery Servers that should be the last resort.

Why are you trying to reinstall macOS Sierra?

Does your mac have a working operating system?

If you do have an operating system on your mac and it is functioning then you can download

Sierra using this method.

How to upgrade to macOS Sierra - Apple Support

Go to Section 4 and click on Download macOS Sierra.

This downloads InstallOS.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running Sierra,

a mac that came preinstalled with an OS later than Sierra will refuse to do the next bit.

When downloaded open to get InstallOS.pkg, double-click on

that and an installation window will open, read and follow the prompts,

this does not install Sierra but uses the InstallOS.pkg to create the 

Install macOS Sierra.app which you will find in your Applications folder.

(If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install Sierra on, that is for later.)

To start the installation of Sierra double click on the Install macOS Sierra.app.

The copy of the install app self deletes after installing Sierra, so make sure you keep a copy of the InstallOS.dmg

if you need it in future, or you could just make a copy of the Install OS X Sierra.app prior to installing and moving

it to an external drive for safe keeping. It is also possible to create a bootable USB stick installer, which once created

you can boot to and perform the same tasks as you can from Apples Recovery Servers without the hassle.

There is a problem with making a bootable USB installer with the Install macOS Sierra.app

where it returns an error Volumes/<USBname> is not a valid mount point. There is a workaround.

With Install macOS Sierra.app in your Applications folder open Terminal.

Paste in this command

sudo plutil -replace CFBundleShortVersionString -string "12.6.03" /Applications/Install\ macOS\ Sierra.app/Contents/Info.plist

press return

enter your password

press return

now paste in the createinstallmedia command for Sierra where MyVolume can be replaced with the name of your USB stick

If your USB name has more than one word separated by spaces you would need to type it in as

/The\ USB\ Name

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app

press return 

follow the prompts

Yes you can use another mac, but as my instructions say it has to be a mac that is capable of running macOS Sierra,

a mac that came preinstalled with an OS newer than macOS Sierra will refuse to convert the InstallOS.pkg to the

Install macOS Sierra.app.