I understand you are using the USB stick to install trisquel.That error almost certainly happened because something is wrong with the image on the usb key.
Verify the iso you downloaded - in terminal - md5sum nameoftheiso.iso See if the output of the md5sum is correct (otherwise re-download the iso)If it is correct - use unetbootin to make the bootable usb stick - select ubuntu 14.04 live in the unetbootin menu..
Try this procedure and see if it works for you byezzz!!
