When you attempt to launch a new instance in the Nectar cloud, the launch may fail with the error messages "There are not enough hosts available" or "No valid host was found" This is a fairly common occurrence.
What does it mean?
The normal meaning of this message is that the availability zone that you selected is "full". The selected AZ does not have enough free resources to launch an instance of the size that you requested. If you didn't specifically select an AZ, it could mean none of the AZs that you have access to have enough free resources.
It can also mean that you have selected an AZ where the resources are restricted to particular classes of project, and your project is not "tagged" appropriately. For example, University of Melbourne currently have a UoM private AZ that is restricted to UoM local projects. If you attempt to launch there and your project is not tagged, you will get the "not enough hosts" error.
It typically does not mean that Nectar, or a specific Nectar node is broken, though node problems or operational restrictions may cause this on occasion.
What can I do about this?
The general approach is to try again:
If trying again doesn't help, raise a support request. They will typically pass your request to the Node operators to see if they can free up some capacity. However, you may be disappointed, or you may have to make do with a smaller instance than you wanted.