The Pokémon Company has sent out a note regarding an issue with Friendly Competitions in Pokémon Sun and Moon. When a player scans a QR code for a Friendly Competition before the competition actually starts, the issue in question sometimes removes players from eligibility to enter the competition and prevents them from playing when the competition begins. The Pokémon Company is currently investigating the cause of this glitch and will soon share a solution to those affected. In the meantime, it is recommended that players don’t scan the QR code for a Friendly Competition until it actually begins.
Source: Serebii
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