What to when you have errors message during macOS Sierra reinstall saying : An error occurred while preparing the installation. Try running this application again

Try Option-Command-R for the latest version of macOS.

Use macOS Recovery on an Intel-based Mac - Apple Support

Start up your Intel-based Mac in macOS Recovery

1. Restart your Mac.
2. Immediately press and hold one of the following key combinations until you see the startup screen:

• • Command-R: Start up from the built-in macOS Recovery System. Use this key combination to reinstall the latest macOS that was installed on your system, or to use the other apps in macOS Recovery.
  • Option-Command-R: Start up from macOS Recovery over the internet. Use this key combination to reinstall macOS and upgrade to the latest version of macOS that’s compatible with your Mac.
  • Option-Shift-Command-R: Start up from macOS Recovery over the internet. Use this key combination to reinstall the version of macOS that came with your Mac or the closest version that’s still available.

1. If you see a lock, enter the password for your Mac.
2. If you have multiple volumes on your disk, select the volume you want to recover, then click Next.
3. If requested, choose an administrator account, click Next, enter the password for the account, then click Continue.
4. When the Recovery app appears in the menu bar, you can choose any of the available options in the window or the menu bar.

What year model is the iMac?

Identify your iMac model - Apple Support

Depending on the year model and if the erased drive is formatted correctly, you should be able to install either the original macOS or the latest macOS available for that year model.

How to reinstall macOS - Apple Support

The only reason to go all the way back to Sierra, is if you were planing to sell or give away the iMac.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Otherwise you should install High Sierra or later.

How to download and install macOS - Apple Support