Why does my iPhone 6 say an error a cured while loading ur software Update

Bearing in mind that an iPhone model 6 cannot support iOS versions past 12.5.6 (because of hardware limits), 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, no low power mode are admitted). AND I suggest that you keep the device plugged into power during the whole update process.

If you don't have a wifi network with Internet access, "borrow" one from your relatives/friends/neighbors, or ask for help at an Apple Store or Authorized Service Provider or Authorized Reseller.

Then re-read carefully the support article that led you here, and do not forget to create a full backup of your device, before any updates.

Regards

Giulio