I can’t update the latest software for Mac desktop. I still have Mac High sierra

What model year of Mac desktop do you have exactly? That will dictate what macOS version you can update to if any.

Check Apple Menu->About this Mac for that information, and post back so we can tell you what you may be able to update to if anything.

To update to a newer software you have to see if you need newer hardware.

If you find you have software which needs 10.15 or newer, make sure none of your software is still in the older 32 bit format before upgrading.