Failed to create unprivileged instance (ubuntu 18.04)

After configuring ~/.config/lxc/default.conv, I meet the following errors when creating containers. Did anyone have similar issue before?

➜  ~ lxc-create -t download -n u1 -- -d ubuntu -r bionic -a amd64

lxc-create: u1: conf.c: chown_mapped_root: 3226 lxc-usernsexec failed: No such file or directory - Failed to open tt
lxc-create: u1: tools/lxc_create.c: main: 327 Failed to create container u1

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