Under vCD 5.1.1 (5.1 also?), creating a new provider VDC will also automatically create a VXLAN network pool.
If you don't have VXLAN configured, you'll get an error - that's the "Ensure the infrastructure is prepared in VSM" error you posted.
There's no way to disable this automatic creation, or to disable VXLAN, so you're stuck with a VXLAN network pool that's constantly showing an error.
Provider VDCs that you created before 5.1 don't automatically get a VXLAN network pool - only new ones created under 5.1.
This won't keep you from creating a new pVDC, but you WILL be stuck with the error.
As an earlier poster said, it sounds like your Org VDC is disabled.