ubuntu installation from usb crashes

[Hèctor Alòs i Font]

I understand very little about all this, so I can't help much. I'm trying to install Ubuntu alongside with Windows. I reserved 50 GB from the disk (seemingly there is just one physical disk) and I tried to install Ubuntu from a USB. Everything goes fine until, as far as I understand, the installer tries to install the grub-installer programme. At this point the installer crashes. I've tried a second time, and I've got the same error.

lsb_release -rd
Description: Ubuntu 18.04.1 LTS
Release: 18.04

apt-cache policy pkgname
N: Unable to locate package pkgname

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.6
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CasperVersion: 1.394
Date: Thu Jan 24 20:14:41 2019
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash ---
LiveMediaBuild: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)