Summary

It is not possible to complete a sprint when one or more issues have Sub-tasks whose current status is not mapped to the last column of the board (on the far right):

That's because the application only consider issues as being completed within a sprint when their current status is in fact mapped to the last column.

The message states the following: Sprint cannot be completed as there are incomplete subtasks on the following issues

The wording might be a little confusing for users when Sub-tasks are in Done type statuses (green ones), which they consider as being complete (end of the sub-task's life cycle).

Suggestion

Change the wording in that message. Maybe make it explicit that the current status of the Sub-tasks is not mapped to the last column.