Unable to update iMac to MacOS High Sierra 10.13

Perhaps when you get an actual downloadable installer, you may need to start up

the Mac in Safe mode; then make the installer run. That might turn-off some other

background issue, that may hinder the downloaded installer's use.

Intel processor, Safe mode on restart:

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac. 
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.

[..And if you use 'Software Update' in your Sierra macOS, that might not work;

Sometimes the control panel, may try offering an Update; instead of Upgrade.]

A matter of knowing IF 'Safe mode' is really booted; has been a problem for some.

Learning how to restart a Mac into safe mode, with advent of Apple Silicon; now

makes reading instructions more effort. Old Intel-based Mac info, didn't change.

How to download and install macOS - Apple Support

//support.apple.com/en-us/HT211683

"macappstores://apps.apple.com/app/macos-high-sierra/id1246284741?mt=12"

The macappstores: 'downloadable installer' on linked-webpage is Active, when

gotten from that site page: And use the Safari browser in your present system.

Use Disk Utility to repair your startup disk. Then try updating or installing macOS again.

Or you may need to reinstall the macOS, & tentatively, try to use downloadable installer.

There are some pitfalls or gremlins, that seem to run amok; try to outwit or humor them.

See my reply on this: macOS High Sierra Full Installer Download… - Apple Community

1. Start up from macOS Recovery

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

APFS won't work for Sierra, Sierra needs MacOS Extended Journaled. High Sierra won't work for APFS on earlier Macs.

What has often worked for others is to use this Restore Method instead of CMD+r keys...

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

2017 iMac failing to install Sierra in re… - Apple Community

Clean Install of High Sierra: Multiple Fa… - Apple Community

Try downloading High Sierra from this link.

How to download and install macOS - Apple Support

The Software Update functionality was only introduced starting in macOS 10.14 Mojave

From your description, you may have visited Apple - Support - Downloads (CA) which only supplies updates to Existing Installation of a Full Version of the OS.

Would suggest using the link provided by a previous contributor to your question.

It should bring down the Full Version of High Sierra macOS 10.13.6

Then and only then commence the installations of the available Security updates

Suggest having at least 50 GB of Empty Space on the drive.

It will allow the Full Version ( about xx.xx GB ) to download, expand and then commence to install.

If there is Third Party Security Software installed - remove it.

It will interfere with both the download and installation process.

I have the same problem, it's so frustrating.