HELP - Unable to boot using USB Bootable drive on Mac with no recovery partition

MoSafe wrote:

I work for a company with a 100 Mac users. One user had upgraded to Big Sur on her Intel Mac and wanted to downgrade it back to Mac OS Catalina due to application compatibility issues, to which i assisted her. I know the downgrade is a nightmare process and clean wiped entire partition. Now we are unable to perform internet recovery at it keeps failing with 1008F, etc. The downside is that she works from a remote location 400 miles from where i work. After multiple attempts i created a USB Bootable installer for Mac OS X Catalina and shipped to her and even then the situtaion remains the same. It gives an option to install MAC OS Catalina but again jumps back to performing Internet recovery.

This has been going on for 15 days, she switched through LAN and Wi-Fi and also to her Mobile Hotspot but to no avail we were sucessful in doing so. Is there anything else that we can attempt to try to fix this ?

Note: The Macbook is Out of warranty and since we do not have apple stores here, there is a cost associated with even simple diagnosis or "how to" through the service vendor.

Sound like the client is still trying to boot to Internet Recovery and the Network is inadequate (error - 1008F)

Holding the Option key to boot to the StartUp Manager and selecting the USBinstaller

Mac startup key combinations - Apple Support

The bootable USB installer includes a Disk Utility—

 Disk Utility> run the First Aid on your Macintosh HD (and the "Macintosh HD-Data" volume as well if Catalina/Big Sur) If errors are found and repaired, run again until no errors reported.

Disk Utility>View>Show All Devices The best sequence is —

• Volume level

• Container level

• Parent drive

https://support.apple.com/guide/mac-help/use-macos-recovery-on-an-intel-based-mac-mchl338cf9a8/mac

To perform this action will require booting from a Bootable Installer. This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Once that is done read on for preparing the Destination computer  >> Only works on Intel Based Apple Computers.

Extra Special Notation regarding the Touch ID equipped Apple Computer. About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. 

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.