How do I reinstall Mac OS X Yosemite on my IMac From 2008 with error message

Does this iMac still boot normally or into Safe Mode? If so, then download the macOS 10.11 installer from the link I provided previously, but if you cannot download it because the browser is outdated, then download the installer using any other computer (Mac or Windows) and transfer the .dmg file to this iMac where you run the .pkg file within the mounted .dmg file to extract the real installer.

If this iMac cannot boot at all to the internal drive and extract the installer and you don't have access to another Qualifying Mac (2008 to 2015), then you will need the OS X 10.6 Snow Leopard uprade DVD. Ask you neighbor for it, or you will need to try to find one online to purchase.

If you have access to another Mac running macOS 10.10 to 10.15, then you can try using DiskMaker X to make the bootable macOS USB installer.

These are your options if you want to install macOS.

Edit: This iMac can only run up to macOS 10.11 which is outdated. Safari won't work to access most websites. Even the popular browsers (Firefox and Chromium based browsers) are no longer compatible with macOS 10.11. There may be modified versions of these browsers which to allow older operating systems to access today's websites. Personally I would not spend any money on this iMac for this reason, plus the internal hard drive is likely to be worn out or failing. If the iMac can boot to the internal drive, then you can check the health of the drive using DriveDx. There is other option using a non-Apple OS, but it would require learning a new OS.

Greetings Heroictima,

If you are having difficulty reinstalling macOS, begin by following the guidance found here:

How to reinstall macOS

"Other macOS installation options

When you install macOS from Recovery, you get the current version of the most recently installed macOS, with some exceptions:

• On an Intel-based Mac, if you use Shift-Option-Command-R during startup, you might be offered the macOS that came with your Mac, or the closest version still available.
• On an Intel-based Mac, if you use Option-Command-R during startup, you might be offered the latest macOS that is compatible with your Mac.
• If you just erased your entire startup disk, you might be offered an earlier compatible version of macOS.
• If the Mac logic board was just replaced, you might be offered the latest macOS that is compatible with your Mac.

You can also use these methods to install macOS, if the macOS is compatible with your Mac:

• Use Software Update or the App Store 
• Use a bootable installer"

Take care.

You can get macOS 10.10 direct from Apple from outside the App Store by using the instructions in this Apple article:

How to download macOS - Apple Support

Make sure to read the instructions carefully, as there are several steps required to successfully extract the real installer into the Applications folder.

Here is another Apple article with optional instructions for creating a bootable macOS USB installer which can give you more options if something goes wrong during the upgrade:

Create a bootable installer for macOS - Apple Support

Make sure to have a good backup before attempting to upgrade macOS just in case something goes wrong during the upgrade.

FYI, A 2008 iMac can run up to macOS 10.11 El Capitan.

You can use any computer to download the older macOS installers such as 10.10, or 10.11 because they are downloaded from outside the App Store. Once downloaded, you must transfer the downloaded .dmg file to a Qualifying Mac in order to run the .pkg file within the mounted .dmg archive file in order to extract the real installer into the Applications folder.

How much free space do you have on the internal drive? You should always have at least 20GB of free storage space for the normal operation of macOS. You will need about 5GB for the .dmg file, and another 6GB or more free space in order to run the .pkg file to extract the real installer which will take up about 6GB in the Applications folder. If you don't have enough free storage space, then you will need to move files to external media, or delete some files to make enough room. Even if you have 20GB of free space, this free space can quickly disappear depending on what is running on the Mac.

TransMac will no longer work to make a bootable macOS USB installer. I have seen no one post any success story for over 5 years now. The instructions I have provided in this thread will work as long as you have a Qualifying Mac. Or you will need to use your OS X 10.6 Snow Leopard Upgrade DVD to perform a clean install of macOS 10.6 after which you can use the information provided to upgrade to later versions of macOS.