Can't install Mojave

[Panicof1907]

So! My last previously used iMac was a disaster that had to be returned. Now I have a replacement, and I have a new issue that makes me want to send this one back too! The computer keeps randomly crashing and restarting. It is a 2017 iMac, and the problem MAY be that I used my High Sierra from Time machine, when it could be that only Mojava and up I think is supported on this iMac.

So, I decided to try to do an update. The problem is, this iMac THINKS that High Sierra is all up to date! I can't seem to update the OS. Now here is where things get weirder. When I look at the Mac App store, the upgrades page shows me what apps need to be upgraded, and tells me that I need to update the Mac to a newer version of the OS. But when I click on the link to take me to the Catalina update, or whatever, it just keeps taking me to the High Sierra page?

I am confused here, any ideas? I am about to ship this computer back AGAIN and just go MS Windows.

 

[CoastalOR]

Here is an Apple Support page for downloading and installing older macOS:

How to download and install macOS - Apple Support

Download and install current or previous versions of the Mac operating system on compatible Mac computers.

support.apple.com

The links to download the OS installer Apps work with the Safari browser. Avoid Firefox.

An 2017 iMac can run Sierra (10.12) to Big Sur (11).

 

[pshufd]

The App Store is messed up in High Sierra because it is so old. I have a Late 2009 iMac 27 and the pages it displays are garbled and missing images. The best way to install clean or upgrade is to create a bootable installer USB for the version that you want. Getting the right link can take a little effort. I have a system with all of the installers from El Capitan through Monterey so that I don't have to deal with finding the right download link. If your system isn't stable enough to download and build the installer, try to find another Mac which you can do this on.

 

[KeesMacPro]

First step is to find out if the crashes/restarts are software or hardware related.
So by doing a clean install , supposing the iMac runs without issues, you can eliminate the possibility of a hardware problem.

A lot of OS versions come with a Firmware update .
In most cases this firmware update will not work if you skip OS versions.

Another point is that by restoring from a TM backup from another device, you may have installed HS without a Firmware update.

So to install Mojave or later, I recommend to do a clean High Sierra install by making a bootable usb installer with createinstallmedia.
Once booted the installer first delete and reformat the internal drive to HFS+ and GUID partition before installing HS
After a completed install, the Firmware will be compatible to the OS installed, regardless the previous installs.

Once HS is up and running correctly , you can upgrade to Mojave (without a usb installer , just with the internal drive).
Always update to the latest version before upgrading to a newer OS.
After succesfully install Mojave you can step up to Catalina if you prefer that.
Note that after FW updates , your iMac remains compatible with older OS versions (in case you decide to revert to an older OS).

After you've got a reliable working iMac with the OS version of your choice , you can easily use Immigration Assistant to restore all your preferences , personal data etc. from your TM backup.

Here's a link to download the HS installer:

How to download and install macOS - Apple Support

Download and install current or previous versions of the Mac operating system on compatible Mac computers.

support.apple.com

And a link about how to make a bootable usb installer:

Create a bootable installer for macOS - Apple Support

Use an external drive or secondary volume as a startup disk from which to install macOS.

support.apple.com