I accidentally erased my HDD trying to migrate files to my new Mac. How do I reinstall High Sierra on my iMac 2011? I tried CMD R, and CMD OPT R on a reboot but was told "Can't connect to the server".

By any chance did you ever install the EFI firmware update in order to use Internet Recovery? If you did, then you can use the same procedure I posted earlier. If not, then you will need the Software Restore DVDs that came with the computer when it was new. You will also need a working optical drive. This won't enable you to recover any data you were trying to migrate since the drive is erased.

Factory Reset Your Pre-Lion Mac

1. Boot the computer using the Snow Leopard Installer Disk or the Disk 1 that came with your computer.  Insert the disk into the optical drive and restart the computer. After the chime press and hold down the  "C" key.  Release the key when you see a small spinning gear appear below the dark gray Apple logo.
2. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities' menu. After Disk Utility loads select the hard drive entry from the left side list (out-dented entry - mfgr.'s ID and drive size.)  Click on the Partition tab in the Disk Utility main window.  Set the number of partitions to one (1) from the Partitions drop down menu, click on Options button and select GUID, click on OK, then set the format type to MacOS Extended (Journaled), then click on the Apply button.
3. When the formatting has finished quit Disk Utility.  Proceed with the OS X installation and follow the directions included with the installer.
4. If you are planning to sell or give your computer away, then do the following: After you reformat your hard drive and reinstall OS X, the computer restarts to a Welcome screen and asks you to choose a country or region. If you want to leave the Mac in an out-of-box state, don't continue with the setup of the system. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the Setup Assistant will guide them through the setup process.

"Can't connect to the server" generally means your Internet connection is not active. Booting into Recovery often requires you to reestablish your wireless connection to your router's internet connection.

Is your old iMac a Mid-2011 or Late-2011 model?

If the Recovery Volume is still functional, then you should be able to do the following:

Install El Capitan or Later from Scratch

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. When Disk Utility loads select the target drive (out-dented entry w/type and size info) from the Device list.
4. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
5. Set the partition scheme to GUID.
6. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
7. Click on the Apply button, then wait for the Done button to activate and click on it.
8. Quit Disk Utility and return to the Utility Menu.
9. Select Install OS X and click on the Continue button.

Addendum:

Reformat the drive to Mac OS Extended (Journaled.).

Starpeeps wrote:

Tried to reformat as "Mac OS Extended (Journaled) but "Operation failed".

Oh dear. Try formatting it in FAT32 format first, and then Mc OS Extended (Journaled)

I've been able to format discs that won't initially format that way before.

I don't know what that means. I need full details on what you did up to the point where that error occurred.

Guess I should have put that in bold. LOL