BootCamp Issue

Scorched Earth Method 

This method will WIPE ALL Data and there is No Recovery - Period.

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example Only : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

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. This applies to macOS 10.14 Mojave and above. 

5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled 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.

Start up your Mac in macOS or Windows with Boot Camp

Start up in macOS from Windows

In Windows on your Mac, do one of the following:

• Restart using Startup Manager: Restart your Mac, then immediately press and hold the Option key. When the Startup Manager window appears, release the Option key, select your macOS startup disk, then press Return.
• If you’re using a Mac notebook computer with an external keyboard, make sure you press and hold the Option key on the built-in keyboard.
• Restart using Boot Camp Control Panel: Click  in the right side of the taskbar, click the Boot Camp icon , then choose Restart in macOS.

Find an Apple Authorized Service Provider or Visit a Genius at an Apple Store

May or may not be costs. If there are costs, Only They would be able to answer that specific Question.

I would be speculating, which is frowned upon on these Forums.

You are welcome and thank you too.

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional questions may arise

Trust you have found another Qualifying Apple Computer to make the Bootable Installer ! ?