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There are no resources available who can be assigned with the epics (e.g. no teams/team members are defined or insufficient team velocity to fit anything in)
The epics don't have an estimate (an epic can have an estimate by itself while it has no child stories, otherwise it is the total estimate of the child stories)
If the epics cannot be scheduled, they are highlighted in red in the backlog table after pressing "Recalculate", and when hovering over the items title (the first column), there is a tooltip giving more details on why it could not be scheduled.
Let us know what you find and we'll go from there.