My apologies then. Because you have correctly tagged this question as LXC. My comment and instructions were about using LXD. That lxc command that you used is part of LXD and cannot be used in LXC.
If you do not have a specific reason to use LXC, I suggest that you look into using LXD as it is simpler and more user-friendly than plain LXC. See more about the two at Comparing LXD vs. LXC
I am not familiar with LXC, therefore I cannot help you much with it.