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Iirc, Ranked Battles only allow Pokemon with the Galar symbol (looks similar to this but a silhouette) so if the symbol were anything else it would reject it. Quite a baffling restriction but they've been doing this since Gen 6.
Now, transferred Pokemon can join Ranked Battles if your talk to the NPC in Battle Tower but with the condition of forgetting their moves. 😔 That's why I still can't enter my Espeon (transferred from Crystal VC and knows Zap Cannon) in Ranked Battles. I don't want to make him forget Zap Cannon!