I just ran the following on command on Ubuntu 20.01.01 with LXD 4.0.4.
lxc copy alpine a2
And got the error
Error: Failed to run: zfs set quota=5000000000 lxdpool/containers/a2@snapshot-alpine-20201221-02: cannot set property for 'lxdpool/containers/a2@snapshot-alpine-20201221-02': this property can not be modified for snapshots
I recreated this issue in the 4.0/stable channel however it looks like it has been fixed in the 4.0/edge channel and so will be in the next 4.0 LTS release which is due in January. The fix also appears to be in LXD 4.9 latest/stable channel.
You can switch to 4.0/edge by running:
snap refresh lxd --channel=4.0/edge
Please keep in mind this channel is a pre-release channel and may contain changes that could prevent a downgrade back to version you’re currently on.