When downgrading from Big Sur to Catalina you need to first of all erase the Disk.
It looks like you did not do this properly.
Boot to your Recovery HD.
Open Disk Utility.
Click View in the menubar and select Show All Devices.
Now you will see the Disk with the Volumes indented underneath.
Highlight the Disk, usually called Apple SSD.... or similar.
Do not select any Container Disk or Volume (Macintosh HD, Macintosh HD- Data or other).
Click Erase.
Give the Disk a name.
Format: APFS
Scheme: GUID Partition Map.
Click Erase.
When Done quit Disk Utility.
Now reinstall Catalina.
The above procedure will erase any and all data on theDisk and its Volumes so make a back up before hand.
When ready only restore your personal data from the backup you made earlier.