The sysctl mentioned in the Debian wiki does not exist in the Linux kernel.
It is provided in a Debian-maintained patch in Debian kernels for the express purpose of disabling user namespaces until they are explicitly enabled by setting the sysctl.
This Debian-specific patch has been refused by the Linux kernel developers.
Because you are not using a Debian provided kernel, user namespaces are always enabled and you do not need to set a sysctl to turn them on. You can simply skip this step.