If a volume has snapshot the system will not allow you to delete the volume, the steps that you need to follow should be as follows;
Click on the ''Compute'' tab on the left side of the panel and select '' Volumes ''. You see '' Volume Snapshots '' which on the top of the screen. You can see all volume snapshots created in this section.
You need to delete the volume snapshot of the volume you want to delete. In the '' Actions '' tab you will be able to click on '' Delete Volume Snapshot ''.
Now you can go back to 'Volumes' section and chose the volume you want to delete then you can click the 'Delete Volumes' button that you see it in the upper right corner.