Its my understanding that, if your save-game's league setup doesn't include a team which it would be appropriate for a Sunday League experience manager to manage, then the UI disables Sunday League from the drop-down.
For example, if your save-game included only E.P.L. clubs, you might see several of the UI options disabled.
This is to minimize the chance that you start out Unemployed with a manager who cannot possibly get a job because his reputation is too low for the leagues/clubs selected in the game.
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