Hi,
Same issue here:
Have a look https://forum.ubuntu-fr.org/viewtopic.php?id=362676 (https://forum.ubuntu-fr.org/viewtopic.php?id=362676)
sudo apt-get install libx11-dev
If it does not work. Do an update then try.
cd /tmp/
wget -c http://fr.archive.ubuntu.com/ubuntu/pool/main/libx/libx11/libx11-dev_1.2.2-1ubuntu1_i386.deb
sudo dpkg -i ./libx11-dev_1.2.2-1ubuntu1_i386.deb
Try with lowercase package name
sudo apt-get install libx11-dev
From : http://askubuntu.com/questions/131688/unable-to-locate-package-libx11-dev (http://askubuntu.com/questions/131688/unable-to-locate-package-libx11-dev)
Worked for me. Ubuntu 16.04