What are external sprints in your plan?

If sprints on your board show with an EXT. lozenge, it means that it’s coming from a board that’s not an issue source in your plan. You can assign issues to these external sprints, but they can’t be used with the more advanced sprint management tools like capacity planning.

To remove the EXT. lozenge and unlock the capacity planning features for this sprint, add the board on which that sprint lives as an issue source to your plan.