If you're having trouble getting a copy of the Catalina installer, try this alternate way:
1. Download dosdude1's free "Catalina Patcher" from here:
We ARE NOT going to "patch" anything, please keep reading.
Launch the patcher
Click continue, and then click continue one more time to get past the "contributors" pane.
Now click "DOWNLOAD A COPY".
(I didn't go past this point).
This should download the Catalina installer. Not sure if it's a .dmg file... or what.
That's really all you needed the "Catalina patcher" for, so quit it.
2. Now you need a tool to create the bootable installer.
I recommend the free "Install Disk Creator", which you can get here:
What’s needed for a clean install on a Mac A backup of the disk before it’s erased. Two options are to use Duplicate (a free utility which can copy and paste an entire bootable volume), or Mac Backup Guru (a paid, fully featured backup utility with unique incremental backups capability) for...
macdaddy.io
With this app it takes only a few clicks of the mouse to create the bootable USB installer.
You need a USB flash drive 16gb or larger.
Since Monterey is "newer than" Catalina, you're going to have to ERASE THE ENTIRE INTERNAL DRIVE to install it.
So... you had better be backed up before you go further.
OK, I'll outline the procedure if you don't know it already:
Power down, all the way off.
Connect the USB bootable flash drive.
Hold down the OPTION key and KEEP HOLDING IT DOWN.
Press the power on button (keep holding option down).
This should bring you into the Startup Manager.
Select the flash drive and hit return.
The Mac should boot from the flashdrive.
If it opens to the OS installer, don't run it yet.
You need to quit it and then select disk utility instead.
Get disk utility open.
VERY IMPORTANT STEP: go to the "view" menu and choose "show all devices".
(you can't see the internal drive unless you do this)
Now, look at "the list on the left"
The top line should be the internal SSD.
Click on it and click erase.
Choose "APFS, GUID partition format".
The erase should take only a few seconds.
Then, quit disk utility and re-open the OS installer.
Start clicking through.
The Mac will reboot one or more times, and the screen may go dark for a minute or more with no other indication of activity. Be patient.
When the install is done, you should see the initial setup screen (choose your language).
At this point,
if you have a backup, connect it and then start clicking through.
Startup assistant will ask if you wish to migrate from another drive, yes, you do.
Point the way to the backup, and give SA time to digest everything.
You can migrate everything, or UNcheck some things. This is UP TO YOU.
The migration may take a while.
When done, you should see your old login screen -- so log in and look around.
Good luck.