Click the Downloads (Laz+FPC) link on the left panel of this forum.Then click on the Lazarus Linux amd64 DEB link.This will download a file named lazarus_1.6-0_amd64.deb to your downloads directory.Once the file has downloaded successfully, double-click it, and supply your password.
project1.lpr(9,3) Fatal: Cannot find Interfaces used by Project1. Make sure all ppu files of a package are in its output directory. ppu in wrong directory=/usr/share/lazarus/1.6.2/lcl/units/x86_64-linux/gtk2/interfaces.ppu. Clean up package "LCL"..
I would recommend fpcupdeluxe (http://forum.lazarus.freepascal.org/index.php/topic,34645.0.html) as the best method for Linux Mint which worked for me.
You should have uninstalled your previous attempts first.Do that and also remove the local config dir ~/.lazarus.Then install DEB packages for FPC, FPC sources and Lazarus.
How did you create the project? After a clean install, if you do Project -> New Project -> Application... then press F9 (Run) but Interfaces.pp still cannot be found, then I don't know what is happening.
@wpThere was a problem with a bootstrapper with fpcupdeluxe.Trunk has a makefile that mentions 3.0.2 as a valid bootstrapper, but there is (yet) no 3.0.2 !Temporary fix available:https://github.com/newpascal/fpcupdeluxe/releases/tag/v1.2.0a