Old iMac (Core i3) cannot download to install macOS High Sierra

Are you able to boot into Recovery Mode (Command + R) or better yet Internet Recovery Mode (Command + Option + R)? This part should still be possible AFAIK. I do know that actually selecting the "Install macOS" option once you reach the recovery mode screen is where the trouble usually begins since Apple has been having issues with this part over the past year (we actually thought Apple disabled those older online installers), but we do still hear about issues from time to time for that stage of the process.

It really helps to know exactly at what stage the process fails and the exact error message you encounter. Booting into Internet Recovery Mode requires a good network connection, otherwise you will encounter a large negative error code (could by your home network, router, modem, or your ISP, or local Internet, once in a while Apple's servers although the latter is rare).

Otherwise you will need access to another Mac which is compatible with an OS your old iMac can use in order to create a bootable macOS USB installer. For example, if your iMac is compatible with macOS 10.11 El Capitan, then you generally have these options for another compatible Mac (there are some exceptions):

• For macOS 10.13 -- a Mac from Late-2009 to 2018
• For macOS 10.11 -- a Mac from 2007 to 2015

That is just a very generalized list, but you can confirm compatibility by using the information in the following article:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

Keep in mind the other Mac can currently be running any version of macOS 10.6+.

If your iMac originally shipped with installation DVDs from the factory, then you also have the option to use an OS X 10.6 Snow Leopard upgrade DVD if is shipped with OSX 10.5 or earlier, or you will need the original gray recovery DVD containing OSX 10.6.x Snow Leopard. After installing OSX 10.6, then you will need to install macOS 10.11 (probably need to use another computer to download the install & transfer to the iMac) followed by upgrading to 10.13.

To install macOS 10.6 to 10.12, you will need to partition & format the whole physical drive (or the Fusion Drive if equipped) using the instructions in the following article since those older versions of macOS do not understand the new drive layout & APFS file system utilized by macOS 10.13 High Sierra.

https://eshop.macsales.com/tech_center/formatting/Mac_Formatting_6-10_R3.pdf

FYI, you should always provide the exact model of the device when asking for assistance online since it is always helpful & many times needed to provide more specific assistance. If you cannot boot the Mac to retrieve this information, then you can enter the system serial number on the check coverage page here:

Check Your Service and Support Coverage - Apple Support

Which exact model is your "very old iMac"? The earliest iMac model that can run macOS High Sierra is the Late 2009 one, but came with an Intel Core 2 Duo CPU. The earliest one with an Intel i3 would be a Mid 2010 model. I am guessing you have the latter one.

I suggest starting with the following Apple Support article: How to download and install macOS - Apple Support

Note that one of the options in this article, creating a bootable installer, will require a working Mac.

Just curious about the exact error message you are receiving.

Is the system date & time correct? I know that having the incorrect date & time can prevent macOS from installing because the security certificate will become invalid. You can set the system date & time using the Terminal app located on the Utility menu on the menu bar. Here is the template you can use to configure the date & time:

date  -u  mmddHHMMyy

Where "mm", "dd", and "yy" are the month, day, and year. While "HH" and "MM" are hours & minutes in 24 hour military time. Each value must be two digits, so add a leading zero.

For example to set the date & time to Jan. 2, 2024 @ 11:33am, the command would be:

date  -u  0102113324

Otherwise you will need to use one of the other methods I listed to install macOS. Unfortunately A1312 is a series designation which covers multiple models from Late-2009 to 2011, but they are all 27" models. If you want to install macOS 10.6.x Snow Leopard, then you will need the original gray restoration DVD which originally shipped with the iMac from the factory.

Try Internet Recovery holding down the keys:

Shift, Option (⌥), Command (⌘) & R (all at once), to load the operating system which came with your Mac; then upgrade from there.