Mobile Data in Settings Screen just show error

I'm sorry to hear that your daughter is experiencing issues with her iPhone 12's mobile data. Here are some steps she can try to troubleshoot and resolve the issue:

Ask your daughter to do the following...

1. Sometimes, a simple restart can fix connectivity issues. Have your daughter restart her iPhone by pressing and holding the power button until the "slide to power off" slider appears. Slide it to turn off the device and then press and hold the power button again to turn it back on.
2. Make sure the iPhone is running the latest version of iOS. To check for updates, go to "Settings" > "General" > "Software Update." If an update is available, install it.
3. Occasionally, a carrier settings update can resolve data connectivity issues. To check for a carrier settings update, go to "Settings" > "General" > "About." If a prompt appears for a carrier update, follow the on-screen instructions.
4. If the issue persists, try resetting network settings. This will reset all network-related settings to their default values. To do this, go to "Settings" > "General" > "Reset" > "Reset Network Settings." Note that this will also remove Wi-Fi passwords, so your daughter will need to reconnect to Wi-Fi networks afterward.
5. Check if the SIM card is properly inserted and not damaged. If it's a new SIM card, it might need activation, and your carrier can assist with that.

Meanwhile you may contact the carrier's customer support for assistance. They can check if there are any known outages or issues with her account.