I think it says that if your opponent has not created any team for a specific league, so you need to save a team for each league to be eligible for each league.
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actually even without a party you get a recommended one or create one after accepting the duel
perhaps opponent has no pokemon over 2500 CP
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You mustn’t use mons over 2500 in that league. 3 Pidgeys with CP 10 are allowed.
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We are talking about Master league, a team of 3 10cp pidgeys would I not likely consider...
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My wife's account has 2 mon over 2500 and isn't eligible to battle me in the master league. Perhaps it's a minimum of 3.