Problem installing Ubuntu on my Mac

Can you boot from the USB Installer of Ubuntu and if so, use the GPart Utility in Ubuntu to Reformat the Entire Internal Drive to FAT. Once that is done, may need another Bootable Installer from Apple with the qualifying version of macOs. Then boot from the macOS Bootable Installer and again reformat the Entire Internal Drive to match the filing System of the macOS Bootable USB - that being APFS / GUID of Catalina and above or HS / GUID if Sierra. High Sierra and Mojave, will depend if Internal is SSD or Rotational drive. If SSD - then APFS if Rotational HFS

You may not actually need a macOS installer drive -- try holding Command + R + Power while booting. Wait until below the Apple logo it says "Loading boot options..." -- from there, use your keyboard's cursor keys to navigate. Highlight "Options" and press Enter. You will be presented with a variety of tools, one of which is "Install macOS (mOS version)". First however, you'll need to open up the Disk Utility and erase your disk.

You may not actually need a macOS installer drive -- try holding Command + R + Power while booting. Wait until below the Apple logo it says "Loading boot options..." -- from there, use your keyboard's cursor keys to navigate. Highlight "Options" and press Enter. You will be presented with a variety of tools, one of which is "Install macOS (mOS version)". First however, you'll need to open up the Disk Utility and erase your disk.

One of the following should work for you:

Boot back into Internet Recovery, specifically using Shift-Option-Command-R (if you are on an Intel-based Mac) and in Disk Utility select the View menu and choose Show All Devices. Erasing "Macintosh HD" is not sufficient, you need to see the whole physical drive and erase that as that will also rebuilt the various hidden partitions necessary for the installer to see a drive.

On the 1% chance this still doesn't work you should erase/repartition the drive as an old-style Windows-style Master Boot Record drive + using FAT32, then immediately turn around bring it back to a GUID partition + using MacOS Extended Journaled (or APFS). Doing so will completely flush out the boot sector and eliminate all boot-record traces of Ubuntu and get the drive recognized by the Reinstall option.

Even in Internet Recovery Mode - the DU can not see the drive at all. This is because it was formatted using the Partition Map ext4 which macOS does not understand. Thus, the need to format to fat32 using the GPart utility in Ubuntu

The disks are probably formatted in ext4. You should open the disk partition tool and completely erase the disks, and then make a new APFS+ filesystem.