how do i unlock the disc to restore my imac

1. In the menu bar, choose Apple menu > Restart. As your Mac restarts, hold down the Command and R keys until the macOS Utilities window appears.
2. Select Disk Utility, then click Continue.
3. Select your startup disk on the left, then click Erase.
4. Click the Format pop-up menu, choose an APFS format or Mac OS Extended format, enter a name, then click Erase.
5. Important: Erasing the disk removes all the information from the disk. Be sure to back up the information you want to keep to an external device.
6. After the disk is erased, choose Disk Utility > Quit Disk Utility.
7. Select Reinstall macOS, click Continue, then follow the onscreen instructions.

Erase and reinstall macOS - Apple Support

It appears you are booting from the internal drive which only has a macOS installer volume on it. You cannot delete or erase that drive while booted from it. You can try booting into Internet Recovery Mode (Command + Option + R) if your Mac is supported so you can access the online macOS installer.

Otherwise you will need to create a bootable macOS USB installer using the instructions in this Apple article:

How to create a bootable installer for macOS - Apple Support

You will need access to another Mac which is compatible with the OS installer you are trying to create. However, the macOS 10.12 Sierra installer is currently broken for this purpose although there are instructions to fix it since Apple has gone out of their way to hide the macOS 10.12 installer instead of fixing it here (I don't have the link handy). You can use this article to see which versions of macOS are compatible with various Apple hardware. For example to create a bootable macOS 10.11 USB installer will generally require a Mac from 2008 to 2015 although that Mac does not need to be currently running that same OS. For creating a bootable macOS 10.13 USB installer a Mac from 2010 to 2018.

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

Sometimes that "OS X Base System" volume may indicate an attempted macOS installation that did not complete. You can try booting the Mac normally to see if it will complete, otherwise you will need to use one of these other methods. If your Mac shipped with DVDs from the factory, then using an OS X 10.6 Snow Leopard DVD would be another option (there are two different types so it is important to know which exact model Mac you have which would be useful information to have here anyway). You can get the exact model by entering your serial number here:

https://checkcoverage.apple.com/

Did you encrypt it?

Have you clicked on the top entry there? The Samsung HDD one?

Is Erase then an Option?

Internet Restore for the original installer is command-option-shift-R unless it is one of the Mx Macs.