This error appears either when the QR code is not properly scanned or if the e-SIM has already been installed.

Sometimes, due to poor connection, QR codes are partially scanned. Try rescanning after ensuring you have good network strength.

To check if your e-SIM has already been installed:

Go To Settings > General > About

Scroll down, and you should be able to see the ICCID and one extra digit if your e-SIM is “on” in your phone’s Cellular Plans. 

Please note that most e-SIM can only be installed once, so if you’re attempting to reinstall a removed e-SIM, it will not work.