Comment 4 for bug 1874878

I had the same issue:
When the Package Configuration dialog came up on Configuring lxd
LXD snap track latest/3.0/4.0
When I chose 4.0 and proceed, the installation hung indefinitely at
==> Installing the LXD snap from the 4.0 track for ubuntu-20.04

My workaround was to do the following at the point the Configuration dialog showed up in seperate shell:
sudo snap install hello-world
sudo snap list

Then proceed with Configuring lxd. The installation proceeded and did not hang at the Installing LXD snap point
$ sudo snap list
[sudo] password for pos:
Name Version Rev Tracking Publisher Notes
core 16-2.44.3 9066 latest/stable canonical* core
core18 20200427 1754 latest/stable canonical* base
hello-world 6.4 29 latest/stable canonical* -
lxd 4.0.1 14890 4.0/stable/… canonical* -