How can I erase and restore an iMac? The Erase option in the disk utility is not available.

Use Disk Utility to erase an Intel-based Mac

How to erase (format) built-in startup disk of an Intel-based Mac.

These steps do not apply to Mac computers with Apple silicon. If you're using a Mac with Apple silicon, follow the steps to erase a Mac with Apple silicon.

Before erasing your Mac

1. If you want to restore your Mac to factory settings, such as to prepare it for a new owner, learn what to do before you sell, give away, or trade in your Mac.
2. If using macOS Monterey on a Mac with the Apple T2 Security Chip, follow the steps to erase all content and settings instead of these steps.
3. Make a backup of any files that you want to keep. Erasing your Mac permanently deletes its files. 

Use Disk Utility to erase your Mac

1. Start up from macOS Recovery: Turn on your Mac, then immediately press and hold these two keys until you see an Apple logo or other image: Command (⌘) and R. 
2. If asked, select a user you know the password for, then enter their administrator password. 
3. From the utilities window, select Disk Utility and click Continue.
4. 
5. Select Macintosh HD in the sidebar of Disk Utility. Don't see Macintosh HD?
6. Click the Erase button in the toolbar, then enter the requested details:

• • Name: Macintosh HD
  • Format: APFS or Mac OS Extended (Journaled), as recommended by Disk Utility

1. Click Erase. However, if you see an Erase Volume Group button, click that button instead.
2. 
3. If asked, enter your Apple ID. Forgot your Apple ID?
4. After the erase is done, select any other internal volumes in the sidebar, then click the delete volume (–) button in the toolbar to delete that volume.
5. During this step, disregard any internal volume named Macintosh HD or Macintosh HD - Data, as well as any volumes in the External and Disk Images sections of the sidebar. 
6. Quit Disk Utility to return to the utilities window.
7. If you want to start up again from the disk you erased, select Reinstall macOS in the utilities window, then click Continue and follow the onscreen instructions to reinstall macOS.

If you don’t see Macintosh HD in Disk Utility

Your built-in startup disk should be the first item listed in the Disk Utility sidebar. It's named Macintosh HD, unless you changed its name. If you don't see it there, choose Apple menu  > Shut Down, then unplug all nonessential devices from your Mac and try again.

If your disk still doesn't appear in Disk Utility, or Disk Utility reports that the erase process failed, your Mac might need service. If you need help, please contact Apple Support.

For more information about using Disk Utility, see the Disk Utility User Guide.

Erasing and reinstalling the OS is a major operation, do you really need to do this,

why is your mac performing poorly, what are the problems you are experiencing.

If the problems are caused by failing hardware resetting your mac will be no help whatsoever.

It may be possible to sort your mac out by examining any reports and noting things that could be

causing your mac to misbehave, could be as simple as uninstalling software which is not kind to your mac,

or just freeing up space.

First of all make a full back up of your mac.

Download and then run this app, https://binaryfruit.com/drivedx there is a free trial period.

If your mac passes this test then you may want to try running this app, https://etrecheck.com

the free version is fine, this will generate a report on your mac which you can post back here using

the Additional Text button on the toolbar at the bottom of a Reply window here.

No personal information is included in the report.

Do not rely on Apples Recovery Servers to reinstall an OS for you, please make sure you have a means

to restore your mac at your own hands.

The first thing to do is download a copy of the Install macOS High Sierra.app.

Do this by clicking on this link, use your Safari browser.

How to get old versions of macOS - Apple Support

Go to Use Safari to download macOS, click on macOS High Sierra.

This will download the Install macOS High Sierra.app to your Applications folder.

Now create a bootable USB flash drive installer using these instructions.

How to create a bootable installer for macOS – Apple Support (UK) After creating the bootable USB restart your mac while pressing and holding the option/ alt

key. In two minutes you will see the Boot Manager, select the USB and press Return.

The mac will now boot to the USB, you will see a Utilities panel.

Click on Disk Utility press Continue.

Click View in the menubar and select Show All Devices.

Highlight the Disk (Apple SSD....... or similar) not any of the indented Volumes.

Give the Disk a name.

Format: APFS,

Scheme: GUID Partition Map.

Click Erase.

When Done quit Disk Utility.

Click on Install OS press Continue.

The installation process will start, follow the prompts.

If you have erased and installed macOS High Sierra you can then think about restoring your personal data from

the backup you made earlier. Only restore your personal data from within your Users/<Useraccount> folder.

Do not restore any system or library folders, as this will put the older perhaps buggy system back on.

Do no restore any apps from the backup ,install them press either from the App Store Purchased page or from

the original installer app/ downloads you kept safe for a rainy day.