I wonder where to put this report but it appears to fit into Development.
First, I’m not happy with how Installation is presented. Option “Replace Partition” does not make great sense to me, “Use Partition” would therefore much more, but it isn’t offered! Also “Add Partition” (manually of a size) would make much sense, but it isn’t offered.
The failure occurred when I chose a partition (20.000 MB, ext4) to be replaced by Manjaro. The partition was removed but nothing new created and installation failed with a message. Environment was BIOS-MBR and the partition was the last logical in Extended and there was a lot of freespace behind it. XFCE 64bit 18.0.3.
I succeeded in the second attempt with creating a new partition in the installation process (not sure any more how I did it).
–> offer “Use Partition” + format choice and simply reformat (what tool are you using?)
–> offer “Add New Partition” + format choice and preferably apply as logical partition
–> in the list of devices (partitions) you could add the device names (extracted from partition table)
Perhaps you can make something out of it.