Had the same issue. Problem was an already partitioned drive (used Debian before on that system). I’ve had to delete all partitions in die disk utility manually. When you see identifiers like /dev/sda1, /dev/sda2, … you have to remove them from the system drive until you see “unallocated space” over the complete volume. The identifier is now /dev/sda (without a number).
I have exactly this problem today. Definitely using “Try Ubuntu” and the restore disk image doesn’t work - wanting user authentication I do not have.
I suspect its something to do with the internal drive I’m trying to restore onto - there’s a partition I don’t know how to delete and I think the authentication request is for permission to wipe that partition. I tried deleting the remnant partition with the Disk utility but same issue - cannot do it without user authentication, which I don’t have in Ubunty try mode.
I think the issue was that I thought the try install had finished - BUT IT HADN’T. There were several other dialog boxes hidden behind the window I was looking at. I think I was too impatient and started using the Try version before the whole install process had finished.
When I rebooted the umpteenth time, I saw the dialog box peeking out, and was able to move to that windows and tick the final “Try” option and this time the internal disk is allowing me to restore the image. Noob error.