Your apt install was a no operation. LXD does not use the apt version, it use its own ‘vendored’ snap version.
The net effect of setting the criu.enable to true and restarting lxd (the logical way would be sudo snap restart lxd, since the change is at the snap level) is to set the path to load the snapified version of criu (that you can find in /snap/lxd/current/criu/). By looking at /proc//environ this can be seen quite easily.
sudo cat /proc/16688/environ
(…)
PATH=/snap/lxd/current/criu:/snap/(…)
Very probably the OP problem was coming from mixing ‘enable’ and ‘enabled’ and not restarting LXD after setting the correct value (enable, NOT enabled). Unfortunately Snap is allowing the user to set any value for a snap, even non existing ones.
sudo snap set lxd foo.baz=spam
sudo snap get lxd
Key Value
criu {…}
foo {…}