I’m having this error at around 80% of the installation which reads as:
Installation Failed
External command failed to finish.
Command apt-cdrom add -m -d=/media/cdrom/ failed to finish in 10 seconds. There was no output from the command.
BTW, this is an Intel NUC, so no CDROM what so ever.
These are just points
the CD refers to your install media you are installing from, which can be a cd/dvd/hdd/ssd/thumb-drive/flash-card/…
did you verify your download? (see https://manual.lubuntu.me/stable/1/1.1/retrieving_the_image.html)
next did you verify your write to install media? (cd/whatever). You’ll find it mentioned in https://manual.lubuntu.me/stable/1/1.3/installation.html but it’s the running “Check disk for defects” which validates the write to your install media was flawless.
This may not be your issue, but these steps are quick and easy to perform, and save hours-days of troubleshooting because of a few wrong bits on the image that may have created troubles.
FYI: If I have trouble with the “Check disk for defects” on a box, I usually use another device to test it, and if it has problems on the second box I assume the install-media is bad (usually a bad write).
Looks like I choose the wrong tool to write the iso from windows to usb, thanks for the support 
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