Download High Sierra

Even though your other macs are running macOS Catalina you should still be able to

download the High Sierra installer app.

Open Safari do not use Firefox and click here,

How to upgrade to macOS High Sierra - Apple Support

Go to Section 4 and click on Get macOS High Sierra

this will redirect you to the App Store High Sierra download page.

It should say Get or Download.

This will download the Install macOS High Sierra.app to your

Applications folder.

With the Install macOS High Sierra.app in the Applications folder and the createinstallmedia

command in Terminal you can create a bootable USB installer.

How to create a bootable installer for macOS – Apple Support

Ok an update I have just switched over to Catalina and guess what, it will not download High Sierra

saying that the OS is not available.

So stupid of Apple to prevent Catalina from downloading Mojave and High Sierra,

but you can download Sierra, El Capitan and Yosemite, there is a lot of not very joined up

thinking at Apple these days. So sorry for leading you up impossible avenues.

For the most part I use Mojave and Mojave downloads High Sierra no problem.

One way to get around it is to install this,

http://dosdude1.com/highsierra/

This downloads a .dmg file which you open.

Control-click on the. app to open it.

Ignore the window that opens but click on Tools in the menubar and select

Download macOS High Sierra

Follow the prompts it will start downloading High Sierra

direct from Apples Servers.

The app is fine and does not pose a threat to your Mac.

If you are unable to recover the standard way follow this How to create a bootable installer for macOS - Apple Support

Use this for the bootable installer - High Sierra download - Get macOS High Sierra

Then finally, follow this link - How to reinstall macOS from macOS Recovery - Apple Support

It most definitely is available, can you share a screenshot of the error.

When you click on 'Get' the mac should open System Preferences> Software Update,

it will search for the download and eventually you should see this,

Whenever downloading anything from Apples servers you should have your mac connected to your router

via cable not WiFi. do not use the mac while it is downloading or any other devices that share the same internet

connection.

Are there no other devices in your household that could be stealing the bandwidth from your

request to Apples Servers. Might be worth restarting the mac you are trying to download

High Sierra to, may clear out any caches that are preventing the download.

Are you using any anti-virus apps or VPN's that might be interfering with access to the

download.

Yeah keep the Auto update unchecked, too often now Apple are releasing updates that contain

problems, only to be re-released a few days later with bug fixes. You can use the Advanced button

and set the mac to inform you of new updates but not install them. Check back here to hear what

people are saying about newly released updates, see if there are problems and maybe wait a week

or so before installing, see if any re-releases have been provided or supplementals added.

Disconnect all other devices that share your internet connection.

You didn't answer whether you are using any anti-virus or VPN.

Open System Preferences> Language & Region see if your mac is set up for your location.

Also check the Date & Time is set for your location and time zone.

And Security & Privacy> Firewall> Firewall Options… check that you haven't blocked anything

there that may prevent downloads from the software servers.

As an experiment see if you can download a free app through the App Store.

Yeah it should be, why would Apple block the download of High Sierra to Australia.

Couple of weeks ago I helped a guy in Brisbane download it.

No point in having AV installed on a mac, there are no viruses that affect the mac,

contact the AV software developer to get full instructions on how to uninstall

the software thoroughly and completely.

Keep on top of malware/ adware by running the free version of, https://https://www.etrecheck.com

https://discussions.apple.com/docs/DOC-8841

If you want you could run https://www.etrecheck.com this will generate a report on your mac,

just use the free version. To post the report back here click on Share Report at the top of the report.

Then use the Additional Text button third from the right of the icons at the bottom of a Reply window

here. No personal information is included in the report.

How good is your internet connection, where are you.

Never have your mac set to Automatically Update.

Yeah the report looks fine, only thing though it has noted that your mac is low on free space.

At 18GBs free it is pretty low it is advised to keep at least 20GBs free at all times, doubt it

would have an influence over downloading High Sierra from the App Store, but if you could

off load files to your external drive to get for example 50GBs free space.

It may be that Software Update is seeing how much free space you have and thinking that

you don't have enough free space if you went ahead and tried to install High Sierra,

Software Update doesn't know you do not want to install it. This theory is a long shot

and doesn't explain why your friend wasn't able to download it unless they have low

free space also.

Have you tried to download High Sierra while in Safe Mode.

Create another user account on your mac with admin privileges and see if it

will allow you to download High Sierra.

Try and find out what the runaway process is, open Activity Monitor> CPU

see if you can quit the process.

Top Processes Snapshot by CPU:

Process (count) CPU (Source - Location)

Other processes 110.57 % (?)

Another thing which is totally unrelated you recently installed Adobe Flash Player,

Safari version 14 no longer supports Flash, and Adobe themselves are stopping

support for Flash at the end of the year.