Robinhood is a customer of Citadel to execute trades. Citadel lent $2.5 billion to the fund that shorted Gamestop. If that fund goes bust, Citadel won’t get its money back. It’s possible they pressure Robinhood to disallow buying Gamestop shares so the price doesn’t increase and the fund won’t suffer a fatal blow. It’s also possible Citadel simply doesn’t accept Buy orders anymore and Robinhood reacted to that by disabling the Buy button for Gamestop shares.