You cannot create Droplets in certain datacenters due to limited capacity. If your snapshot is in a limited capacity datacenter, you can’t create a Droplet from that snapshot.
If you have a snapshot in a datacenter where Droplet creation is not available, transfer the snapshot to another datacenter. You can then create a new Droplet from that snapshot in the new datacenter.
You can see where Droplets are available on the Droplet availability page.