Updating to iOS 14.5.1

I was just on the phone with Apple Support, it's a known issue that affects only users in Australia and New Zealand. They are working on a fix, but in the meantime a workaround is to temporarily set the language to e.g. English US (instead of AU/NZ) and attempt the update again. That worked for me! After the update you can safely set it back to Aussie or Kiwi language again :-)

And what might that be?

???

Connect to a WiFi network, then attempt the update again.

Connect to Wi-Fil

You need a Wi-Fi connection for iPhone updates.