Use git clone https://github.com/resloved/i3.git
Paste the following into /usr/share/xsessions/i3.desktop
[Desktop Entry]
Name=i3
Comment=improved dynamic tiling window manager
Exec=i3
TryExec=i3
Type=Application
X-LightDM-DesktopName=i3
DesktopNames=i3
Keywords=tiling;wm;windowmanager;window;manager;
fix the problem and install i3-gaps:
$ sudo add-apt-repository -y ppa:regolith-linux/stable
$ sudo apt install i3-gaps