Remove Ubuntu from mac

Then you have just two options since you completely wiped out the macOS part of the drive.

Try booting into Internet Recovery Mode using Command + Option + R to access the online macOS 11.x Big Sur.

Unfortunately some Macs may only boot to the online installer for the OS which originally shipped with it from the factory which is Ok, but will affect how you prepare the drive.

The other option is to create a bootable macOS USB installer using the instructions in this Apple article if you have access to another compatible Mac generally from 2007 to 2020:

Create a bootable installer - Apple Support

Generally you need a Mac with its model listed as:

• 2013 to 2020 to create a bootable macOS 11.x Big Sur installer
• 2012 to 2019 to create a bootable macOS 10.15 Catalina installer
• Late-2009 to mid-2018 to create a macOS 10.13 High Sierra installer
• 2007 to 2015 to create a macOS 10.11 El Capitan installer

You can confirm another Mac is compatible with the installer you want to create by using the information in the following article:

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

Now that you have a bootable macOS installer, you need to properly the drive for macOS. This varies depending on the version of macOS you are installing.

• With macOS 10.14+ you want to erase the whole physical drive as GUID partition and APFS (top option).
• With macOS 10.11 - 10.13, you want to erase the whole physical drive as GUID partition and MacOS Extended (Journaled).

Make sure to give the volume a proper name.....the default volume name from the factory was "Macintosh HD". It is tricky to properly change the volume name with macOS 10.15 once the OS has been installed.

Also, with macOS 10.13+ you will need to click "View" within Disk Utility and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility.

I'm assuming there is no Fusion Drive setup here, otherwise just erase the Fusion Drive item instead of the physical drive.

I'm also assuming that there is no third party internal NVMe SSD installed since that requires only using macOS 10.13+.

You will need to delete the Ubuntu partition(s) using Disk Utility within macOS & hope you can merge that space back into the main macOS partition/Container. Unfortunately this does not always work.

You will need to also delete the "ubuntu" folder on the hidden ESP (aka "EFI") partition. While booted into macOS, launch the Terminal app and issue the following command which will mount the hidden ESP/EFI partition:

For a single internal drive:

diskutil  mount  disk0s1 

If your iMac has a Fusion Drive:

diskutil  mount  disk2s1

Make sure you have no other external drives connected as they may interfere with the way the various drives & volumes are identified by macOS rendering my commands above incorrect.

I'm also assuming you were dual booting macOS & Ubuntu from the internal drive and can still boot into macOS on the internal drive.

Use the Finder to view the "EFI" volume where you should see a folder called "ubuntu". Just delete the "ubuntu" folder which contains the Ubuntu bootloader. Eject the "EFI" volume.