This error appears for one of two reasons. Either when the eSIM is installed fully or partially on a device already.
Usually the install will go successfully, even if the device locks it will often continue in the background on newer devices. Rescanning the QR code will give this error often meaning it is already installed. Check through your settings to see if it is already there.
If there is an issue with the install the partially installed eSIM should show in your settings and you will require to delete it to be able to rescan the QR code and start a fresh installation.
To check whether the eSIM has installed successfully you can talk to the chat bot which will check for you in our systems, if it is a yes, you should be able to find the eSIM in your phone settings.