Robinhood had to disable opening new positions, because they are unable to meet their margin requirements. They only survived, because they were granted a discretionary rebate. Otherwise, the DTCC could have taken control of their entire portfolio, and could have liquidated everything to protect its other member firms.
RH had poor internal practices, and did not fully understand how these margin amounts were calculated, nor had they implemented best practices for modelling those. Additionally, they technological infrastructure had already been at the edge of breaking down under the load for days even before the meme stock event.
Citadel Securities had nothing to do with RH not accepting new buy orders. In fact, they were the only one of RH's six market makers who never asked them to route away orders. The only thing the did negotiate for was a reduction of PFOF rebates, because RH was using its own, non-standard approach for calculating those that resulted in RH receiving a huge increase in rebates.