When I try to do the software update it says cannot complete update. Retry does not work

Assuming you're trying to update your device wirelessly, and bearing in mind that to perform an update procedure wirelessly, you need to connect your device to a reliable and fast Wifi Network with internet access (no cellular data, unless you use a 5G network, no personal hotspot on your device, no VPN connections are admitted, and I suggest that you keep the device plugged into power during the whole update process), see this suppport article:--> If your iPhone, iPad or iPod touch won’t update

If the issue persists, try to update it via computer:

Learn how to update your device manually if you're using a Mac with macOS Catalina or later, or if you're using a Mac with macOS Mojave or earlier or a Windows PC.

If you still get no help, contact Apple Support directly, using this link:--> Get Support

Regards

Giulio