Prepping a newly-erased iMac for. new user

When (if) you get the OS installed, immediately dismiss the installer when finished - do not go through the setup process. That way the new owner can set it up with their Apple ID.

And, make sure you do all the other steps here such as signing it out from your account, etc.

https://support.apple.com/en-us/HT201065

On your new iMac, try to create a macOS High Sierra bootable installer: How to create a bootable installer for macOS - Apple Support.

Jack

You will be unable to create a bootable macOS 10.13 USB installer using a 2021 Mac unless you already have a copy of the High Sierra installer already downloaded and stored from an older Mac. If you have access to a 2010 to 2017 Mac, then most of them will allow you to download and create a bootable macOS USB installer. Or if you have access to a 2008 - 2009 Mac, then you can create a bootable macOS 10.11 USB installer. If these are not options, then you will need to use the original OSX 10.6 Snow Leopard DVD which shipped with the iMac from the factory (the DVD will have a gray label which notes the exact model of the iMac).

You can try changing the date on the iMac using the command line. You can make sure the date is current, but if that gives you the same error, then try changing the date to some time in 2017 (and if that fails try a year earlier, etc.). Launch the Terminal app from the Utilities folder while booted to the installer.

Set the date to Jan. 16, 2022 @ 11:33AM:

date  -u  0116113322

Set the date to Jan. 2, 2017 @ 11:33AM:

date  -u  0102113322

Note: the last two digits is the year. Press the "Return" key at the end of each line to execute the command.

It is best to connect the Mac directly to the router using an Ethernet cable for a faster more reliable connection which bypasses hard to troubleshoot WiFi issues.

HWTech wrote:

If these are not options, then you will need to use the original OSX 10.6 Snow Leopard DVD which shipped with the iMac from the factory (the DVD will have a gray label which notes the exact model of the iMac).

Among these suggestions, you can also purchase a retail Mac OS X Snow Leopard Install DVD. Apple no longer sells them, but legitimate aftermarket sources exist. Expect to pay a little more than what Apple used to charge ($19) but not much more. Be sure the box and disc are white with a picture of a snow leopard on it.

Otherwise use the grey disc that came with the Mac, so see if you can find it. Those discs should accompany the Macs that used them, from cradle to grave.

After installing Snow Leopard, run Software Update which will install the App Store. Then, stop. Let the new owner upgrade it to High Sierra because it will require an Apple ID that the new owner should provide. Don't use your Apple ID otherwise you will make life miserable for its new owner.

There was an old problem long since resolved, but considering Snow Leopard is about that old try the following.

Using Disk Utility (an option that ought to be available from the Snow Leopard Install disc's Utilities menu), select the startup disk (the 500 GB internal one) and resize it. The amount doesn't matter. Click Apply after resizing it. At your option, resize it again to match its original capacity. Click Apply again.

Then, attempt the installation again.

I'm relying on memory for this workaround, and also hoping it may evoke similar memories from other ASC helpers, but if you remain stuck please write back and I'll attempt to find some historical references to this particular problem.

Here is how you properly prepare a drive using a macOS 10.6 to 10.10 installer by partitioning and formatting the physical drive:

https://www.owcdigital.com/assets/support/support-formatting-and-migration/Mac_Formatting_6-10.pdf

However, those instructions do not include securely erasing the hard drive so any unencrypted data on the drive can be accessed by the new user using a data recovery app. If this Mac uses a hard drive, then you need to either first enable Filevault, or you can select the "Secure Erase" option and write a single pass of zeroes to the hard drive. This is not necessary if the Mac only uses an SSD. If the Mac will boot into macOS, then it is much easier to first enable Filevault and let the process fully complete before attempting to erase/partition & format the hard drive since the location of the Secure Erase option in Disk Utility has changed over the years.

Welcome to the Apple Support Communities.

How is your 2009 iMac connected to the internet? Do you have access to another Mac?

Jack

First, Erase and be sure to select GUID Partition Map.

Mac OS Extended (Journaled) is the correct choice.