After upgrade 17.10 -> 18.04 I get: Failed to connect to lvmetad

I had this same problem, and it seems to be two problems.

First, there is the error message which has to do with the bug report. It reflects a long boot time that can be shortened using the directions at this question, as long as you do not mind disabling hibernation. It will not remove the error message, but it will dramatically speed up the boot time.

Second, there is the lack of graphical display. This can be fixed by running

sudo systemctl start gdm

This starts gdm3, the Gnome login display. For me, the change persisted, so even after rebooting, I did not need to reenter this command.