What do I do if my macOS Catalina upgrade won't download

Try the following:

1. If not already, temporarily connect your notebook by Ethernet to your router, and then, try upgrading again.
2. If that still doesn't work, boot up your Mac in Safe Mode, and then, try again.

Ref:  Use safe mode to isolate issues with your Mac - Apple Support

What exactly happens when you try to download macOS Catalina? Do you get any error messages?