Problem

As mentioned here, to complete a sprint, all subtasks from a Done parent must also be set to Done before it is possible to complete the sprint.

When the subtasks are complete, however, if any of the subtasks are in a status that is not mapped on the board, the error displayed simply mentions that there are incomplete subtasks that need to be complete:

Failed to complete sprint: Sprint cannot be completed as there are incomplete subtasks on the following issues: <issue-key>

This video shows both how to replicate the problem and how to solve it: CompleteSprint-SubtaskNotMapped.mp4

Why is it important?

Without a detailed error, it is hard to understand why the sprint is not deleted/completed, as the subtasks may already be Done, but the status is not mapped.

Suggested resolution

Improve the error message to a more detailed message on why the sprint can't be deleted/completed.

Workaround

Map the status of the subtask on the board and ensure the subtask is in a Done status.

This video shows both the problem and how to solve it: CompleteSprint-SubtaskNotMapped.mp4