Is Seagate Backup Plus Slim 2TB External Hard Drive Portable HDD compatible with MacBook Air Catalina OS?

Yes as long as you format it with a compatible file system: https://support.apple.com/guide/disk-utility/file-system-formats-available-in-disk-utility-dsku19ed921c/mac

Be careful also if you plan on using it as your internal system disk as performance is not great especially if you format it as APFS.

Yes it should be.

First thing to do is reformat it when you first attach it to your mac.

Open Disk Utility. click View in the menubar and select Show All Devices.

Highlight the Disk not the indented Volume.

It will be called Seagate something or other in the External portion of the

left hand panel.

Click Erase.

Give the Disk a name,

Format: Mac OS Extended (Journaled)

Scheme: GUID Partition Map.

Click Erase.

When that has done your disk is ready for usage.

Do not install any Seagate software that comes with the disk this will only interfere

with the macs own management of the disk.