Instance is not unique

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2

I have another instance of this issue.
I have the following:

mutation CreateARanking {
  createRanking(
    data: {
      active: true
      rankingname: "TestRnk6"
      rankingdesc: "U17"
      player: { connect: "287674108858073605" }
    }
  ) {
    _id
    rankingname
    rankingdesc
    player {
      _id
    }
  }
}

TestRnk6 doesn’t currently exist (AFAIK). It doesn’t appear in the Ranking collection.
Player _id = 287674108858073605 does already exist.

The previous answer (above) to this issue was to remove the unique constraint on allPlayers index.
However, AFAIK, there is no unique constraint on player.
Anyway, no allPlayers index has been automatically generated in the Fauna UI (there is allUsers and allRankings, but neither of them have unique constraints set either).
Player is defined as:

type Player {
rank: Int!
ranking: Ranking! @relation
challenger: User @relation
}

in the schema.
SOLVED, with thanks, by @ptpaterson:

Why does the CreateARanking mutation in PG give a
“message”: “Instance is not unique.”,
error message?
thanks …