Mac os Catalina downloading failed

Start by going to the Applications Folder and Drag and Drop the application called " Install Catalina " to the Trash and then Empty the Trash.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

macOS Catalina 10.15 hardware requirements Does this computer Qualify to run Catalina ?

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may require an update or upGrades to 64 Bit Version

and in addition to P. Phillips' reply, are you attempting the download over wi-fi? when possible, you should perform macOS downloads with a wired ethernet connection. if this is not possible, (mac has no ethernet port) you should move the mac as close as possible to the router, and temporarily remove your other devices from the wi-fi. you need the most bandwidth going to the mac as you can get while downloading a macOS. I would also recommend trashing the current Catalina installer and start over.