That picture does not tell the cause of why bootloader installation failed. But there is a way to get the logs that would tell us the cause. Boot the ISO Live, open Konsole (terminal), and use this copy and paste command:
$ pkexec calamares -d | tee /home/live/calamares-installation-log.txt
That will create the file in home directory called calamares-installation-log.txt which you can attach here. Then we can probably learn what the cause of this problem is.
For any user having difficulties installing OMLx Rock or ROME versions there is a what to do guide here.
Returning to this after a long time, apologies for the delay. To address this issue we would need more information. The information provided tells us that the Calamares installer fails to install the boot loader because of Errno 17 which simply means “File exists error”. It does not tell us why Calamares thinks some file exists that it thinks should not. We need that “why” in order to get to something we can fix. Given that by far most installations are successful it is not at all obvious why one or two users get the “File exists” error. Based on past history each case of “File exists” error is unique and may each have a different solution.
The lines of code mentioned are the lines that trigger the error. They may or may not have anything to do with the cause of the problem. Again based on history it is rare that they actully point to a solution.
This is why we need more information. To resolve this we need to see the entire calamares-installation-log.txt.