Upgrade from Big Sur to Ventura

Firstly suggest make sure the exiting Big Sur is running the lasts Version 11.7.4

Attempting to upGrade to a newer version of macOS is best attempted from a fully up to date existing version of macOS

Links below specific to this issue

Keep your Mac up to date - Apple Support (CA)

What's new in the updates for macOS Big Sur - Apple Support (CA)

In Theory the computer can jump the quay and move directly from MacOS 11 to macOS 13 but may suggest moving to Monterey before attempting the move to Ventura

User's Choice though

Very strange... based on the Ventura compatibility macOS Ventura is compatible with these computers - Apple Support, you should be able to upgrade with no problem. Are you able to go to the app store > search for Ventura and download it that way?

Follow on

There are factors:

1 - Like poor Wifi connection

2 - Slow wifi connection

3 - Too many other people downloading at the same time and Apple Severs are overloaded

4 - Software on the computer interfering with the download process including AntiVirus Software

Notation: Disabling the AV Software may or may not solve the issue. Removal of the AV Software may increase the chances of the download to perform as expected.

5 - Running behind a VPN which may increase Packet losses or corruption of the download 

6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer. 

7 - Best Results are via Ethernet Cable to Router

8 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal and Try Again using the Software Update Applet in System Preferences

Upgard