For anyone under Debian/Ubuntu failing to recompile Lazarus because of laz.virtualtrees.pas not found.
Reason is, if you installed Lazarus using Apt or Synaptic, that lazarus directories and files are owned by root. Change ownership to your user so Lazarus has permissions and the problem goes away:
sudo chown -R your_name:your_name /etc/lazarus/
sudo chown -R your_name:your_name /usr/lib/lazarus/