The "Complete Sprint" button id displaying not active/grey out due to which not able to perform the action. There is only one board available and under which 3 sprints are created.
Make sure all the tasks are done in your sprint and check whether you have the project admin rights are not on the project.
As per Ending a Sprint documentation: "You must have the JIRA 'Project Administrator' permission for all project(s) with issues included in the sprint to complete it. If you cannot complete a sprint, check whether your sprint has issues from a project where you are not a project administrator.
As Rambabu has said, you need to check the board filter by going to Board settings and then General. Look at the filter query there, which may give you some clue.
Taking a step back, have you ever been able to end sprints, or is this something that has suddenly happened? What has changed?
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