Comment 31 for bug 1853506

hah, bingo, the default kernel package for VMs doesn't have nd_pmem or nd_btt. I had to install linux-modules-extra-5.4.0-12-generic.

Now it works:
ubuntu@focal-nvdimm:~$ sudo ndctl create-namespace
{
  "dev":"namespace1.0",
  "mode":"fsdax",
  "map":"dev",
  "size":"1006.00 MiB (1054.87 MB)",
  "uuid":"53defad5-c13c-4f3c-abbb-d9152db43c69",
  "sector_size":512,
  "align":2097152,
  "blockdev":"pmem1"
}

Thanks a lot, Dan!