Y can’t I update over mobile data

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 friends/neighbors, or ask for help at an Apple Store or Authorized Service Provider or Authorized Reseller.

Do not forget to create a full backup of your device, before any updates.

Regards

Giulio