My iPhone is not activating with error "Failed to retrieve configuration"

Did you just get it?

That message typically only comes up on corporate owned devices that are managed by an MDM. I would expect to see that message if the device either doesn't have a connection to the MDM host or has been flagged as missing, blocked, or stolen in the management system

Try and force restart the iPhone to see if the issues persist

For iPhone 8 or later

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button.
4. When the Apple logo appears, release the side button.

for iPhone 7

Press and hold both the volume down button and the Sleep/Wake button at the same time. When the Apple logo appears, release both buttons.

for iPhone 6S or earlier:

Force restart iPhone 6s or iPhone SE (1st generation)

Press and hold both the Sleep/Wake button and the Home button at the same time. When the Apple logo appears, release both buttons.