27 inch iMac "Big Sur" Internal Drive not shown as a Install Location for Catalina

From the description above, the user is attempting to Downgrade from macOS 11 Big Sur >> to macOS 10.15 Catalina ?

The Operating System will not allow installing and Older Version ( Catalina ) over the top of a Newer Version of macOS 11 Big Sur.

It will require Re-formatting the Entire Internal Drive

Scorched Earth Method 

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

>> Only works on Intel Based Apple Computer <<

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 : Bootable Installer of Catalina would have to be done on a computer that Qualifies to run Catalina.

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.

Extra Special Notation regarding the Touch ID equipped Apple Computer.

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.