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"Complete sprint" is usually greyed out when Jira has a reason that the sprint cannot be ended.
In most cases, it is because the sprint has "Schrödinger's issues" - issues that are done and not done at the same time. An issue that has been moved to the "done" column, but still has sub-tasks that are not in the "done" column (It's an easy thing to do that in Jira, unless you have an admin who has seen the problem before and sets up your workflows such that you can't move an issue with open sub-tasks into done)
The second most common cause is that the sprint has not actually been started. There's no way to end a sprint that hasn't started, it does not make much sense to do that!
Check the status of the sprint first, as it will only take you a few seconds. The sub-task thing is a bit more complex to check because it is possible to build poor board configurations that drop issues of some status off the board, so they can never be seen as "done". The instinct is to ask for a filter that says "sprint = X", but that will not tell you if issues have not-done sub-tasks, so have a look at the board filter, and the column mappings instead. If you have any sub-tasks in unmapped status, you need to get them into "done" (last column on the board)
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