Proxy ausschalten und lxd und snap neu installieren. Aufgetreten beim Upgrade von Ubuntu 18.04 auf Ubuntu 20.04.

sudo snap set system proxy.http=""
sudo snap set system proxy.https=""
sudo apt purge snap snapd

rm -rf ~/snap
sudo rm -rf /snap
sudo rm -rf /var/snap
sudo rm -rf /var/lib/snapd
sudo rm -rf /var/tmp/snapd.swn
sudo rm -rf /var/tmp/snapd.swk
sudo rm -rf /var/tmp/snapd.swl
sudo rm -rf /var/tmp/snapd.swm
sudo rm -rf /var/tmp/snapd.swn
sudo rm -rf /var/tmp/snapd.swo
sudo rm -rf /var/tmp/snapd.swp
sudo rm -rf /etc/systemd/system/snapd.service.d
sudo apt install snapd
sudo snap install hello-world
sudo snap remove hello-world

sudo apt install lxd

Quelle:
https://askubuntu.com/questions/1104729/how-to-fix-unable-to-contact-snap-store-error-while-installing-conjure-up
https://askubuntu.com/questions/1106106/how-do-i-fix-my-connection-to-api-snapcraft-io