MacOS won't update because of internet connection, although I have strong internet connection

There are factors:

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

2 - 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.

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

4 - 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

5 - Not sure the Terminal Command line will work in macOS 10.15 Catalina but you can try just the same. Should some type of error message in Terminal appear, that might confirm this suspicion but hopefully not 🤞

designbath wrote:

I'm having issues updating my macOS Catalina to macOS Ventura.

I receive an error message for a poor internet connection, despite the fact my internet is stable.

What can I do to resolve this issue?

Did you try getting on a different Network and compare your results...

designbath wrote:

Yes.

Seems odd a different Network would give the same result...

You can see available software  installers directly from the Terminal.app, copy and paste: 

softwareupdate --list-full-installers

example to download the Ventura, you copy and paste:

sudo softwareupdate --fetch-full-installer --full-installer-version 13.4.1

when complete the installer will be sitting in your Applications folder "Install macOS Ventura.app"

you can launch it from there like any other application.

It doesn't matter if you have a fiber connection, the Internet is a series of hops from your ISP through other network hardware and ISPs. Anything wrong in that connection between Apple servers and you can cause issues with the download process. Somebody in those hops may have messed up their DNS servers, or it could be a denial of service attack…

Talk to your ISP support team and have them check their network back to your edge device (cable modem) to ensure there is no problem there. They may be aware of other network problems they have no control over too.