@Danp I am on XOA 5.98.1 with XCP-ng 8.2 LTS all patches applied. Ran into the same issue. I believe it was caused by a typo when defining the amount of vCPU, typed 42 instead of 4. Although I fixed it (visually was showing 2 vCPU, looks like it did not actually change it). Once I downloaded the error it was clear the reason, however using XOA GUI I could not find a way to fix it:

{
  "errors": [
    "xen22: HOST_NOT_ENOUGH_PCPUS(42, 32)"

The interesting part is that XOA GUI was showing me only 2 vCPU.

I ran XCP-ng Center (20.04.01) on a Windows laptop and fixed it under Properties > CPU (where it was showing 42).

Regards,
ed