This error appears either when the QR code is not properly scanned or if the eSIM 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 eSIM 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 eSIM is “on” in your phone’s Cellular Plans. Please note that most eSIMs can only be installed once so if you’re attempting to reinstall a removed eSIM, that will not work. Check your eSIM app/website for more details.