Cannot delete instances after block device mapping error.

 I am trying to test Cinder and launching an instance using Cinder using Horizon. Steps I did:
Created a New Volume: - gave type name , Size :1 GB,  Volume Source: empty and the availability zone.
Launched an Instance: Gave an Instance name,  availability zone , Flavor -small (20GB Root Disk),  Instance Count -1, Boot Source: Boot from Volume.

The error I got was “Error: Block Device Mapping is Invalid"  // when spawning the Instance

After this point I cannot delete the instance