Why i cannot close a sprint, even though all of my tasks are under the status "closed"

[Nijo S Thanjan]

All the main tasks are under the status "Done". Also some of the main tasks have some sub-tasks, which are also under the status "closed". when I try to close the sprint it is being shown that your sub task is still not closed. But actually all the sub tasks are under the status "closed"

[Jack Brickey]

Do you have the necessary Manage Sprints permissions?