Cannot install OS update. Help!

Until you fix whatever problems you have you should not attempt an update/upgrade.

A Troubleshooting Protocol to Identify Problems or Fix macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Disconnect all third-party peripherals and remove any support software like drivers and plug-ins.
3. Resetting your Mac’s PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
6. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
7. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
8. Download and install the OS X El Capitan 10.11.6 Combo Update or 10.12.6 Combo Update or Download macOS High Sierra 10.13.3 Combo Update as needed.
9. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
10. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continue button.

Hello Miss Sid, as of today over 300 people have reported this exact problem when trying to upgrade to High Sierra, the no.1 reason was that too many people per-say were trying to download the new OS at the same time causing connection hickups affecting the download process. The solution was simply to try again later.

The other reason was the lack of space in the Hard Drive, OS High Sierra requires 15GB for a sure install even though the initial download packages shows to be only 5GB of size.

And the last most common reason was that the computer does not have all the hardware minimum specifications needed for the install.

This are not the only reasons for that error but are the most common ones. Do me a favor and reply with the following info to get started for a customize solution:

1) Click on the Apple logo on the upper right left corner of your screen.

2) Select "About this Mac"

3) You should see in a new window the OS name, for example, "OS X Yosemite Version 10.10", please paste whatever your Mac shows in your reply.

4) You should see as well the model, for example, "MacBook Pro, Retina, 15-inch, Late 2013", please paste whatever your Mac shows in your reply.

5) And finally in this same window click on the "Storage" tab on the top. This will show how much free space is available in your Mac, just make sure that is says more than 15GB free.

Try setting up another admin user account to see if the same problem continues.

You are welcome.