how to reload mac os sierra

What is the exact model of your iMac? You can get this information by using the Apple check coverage page here:

https://checkcoverage.apple.com/

If your iMac is older a 2010 model, then you do not have access to Internet Recovery Mode. If you have an older iMac, then you would need to use local recovery mode assuming you did not erase the hidden recovery mode partition. Use Command + Option + R to access Recovery Mode. If your iMac is older & only supports local recovery mode, then this should still allow you to boot to the local recovery partition if that partition was not erased and it should forego the need to authenticate with an AppleID which is important for selling the computer. If it is a newer model, then this key combination will access Internet Recovery Mode instead.

If Recovery Mode/Internet Recovery Mode are not options, then you will need to access to another working Mac that can run a version of macOS that your old iMac can run (the other compatible qualifying Mac can currently be running almost any version of macOS). This is another reason we need to know the exact model of your iMac so we can know which versions of macOS are compatible with it so we can suggest other compatible qualifying Macs which can be used to create a bootable macOS USB installer.

The goal here is to install any version of macOS onto this iMac. You cannot be picky at this point....plus once this iMac has an OS installed, then you can create a different macOS USB installer for the OS you wish to place onto this system.

Try the workarounds below if your Mac is new enough to support Internet Recovery. Try High Sierra first.

https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/

https://mrmacintosh.com/fixed-an-error-occurred-while-preparing-the-installation-macos-sierra-recovery-error/

myteam wrote:

many thanks for this. my is a 21.5 inch purchased in Feb 2013. it is the one with the cd drive built in on the side. does this make a difference?

There were still two 21.5" iMacs available from Apple in Feb. 2013 according to MacTracker. A Late(2011) model and a Late(2012) model. The latter model is able to run slightly newer versions of macOS than the 2011 model. If you use the system serial number found on the bottom of the iMac base you can use the Apple check coverage link I provided in my other post to get the exact model.

If you have the Late(2011) model, then when booting to Internet Recovery Mode using Command + Option + R, you will likely need to use Fix #3 in the article linked by @Matti Haveri. If it is a Late(2012) model, then I don't think you won't have that issue installing Catalina.

If you need to create a bootable macOS USB installer:

If you have a Late(2011) model, then you will need another Mac model:

• • macOS 10.13 High Sierra -- Late 2009 to 2018
  • macOS 10.11 El Capitan -- 2007 to 2015

If you have a Late(2012) iMac, then you will need another Mac model:

• • macOS 10.15 Catalina -- 2012 to mid-2020
  • macOS 10.13 High Sierra -- Late 2009 to mid-2019

These are just generalizations on the Mac models as there may be a few exceptions. You can use the information in the following article to confirm whether a particular Mac is compatible with a specific version of macOS:

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