User-Agent:       Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
Firefox/52.0
Build Identifier: 

I guess you will need a bug report (and logs) for this.

Installing Leap 15.0 Build 139.1.  The install was to an existing LVM in a KVM
virtual machine.

It complained about my EFI partition being smaller than 256M.  I ignored that
and continued.

Then everything looked good until I told it to start the install.

Error message: failed to mount /dev/system/swap as swap -- device not found
(that might not be the exact wording).

At that point, I used CTRL-ALT-F2, and looked.  "/dev/system/swap" did exist. 
But "/mnt/dev/system/swap" did not exist.  Nothing had yet been mounted on
"/mnt".  It looks as if the installer had checked prematurely.

CTRL-ALT-F7 took me back to the GUI.  I told it continue.  I got a similar
failure message for "/boot/efi" and then another for "/home".  I don't remember
if there was a message for "/".

I told it to continue.  Later, while the install was going on, I used
CTRL-ALT-F2 again.  And by now everthing was mounted as needed.  I created
"/mnt/etc/crypttab" (copied a file I had previously created).  Back to the GUI.
 The install completed without any other problems.  The newly installed system
booted.

Will post yast logs shortly.

Reproducible: Always