What does it mean that Rocket Pool is at 'max capacity'?

Rocket Pool relies on independent operators joining their protocol and using Rocket Pool's software to set up new validators using ETH pooled in the protocol. Sometimes, there may not be enough new validators joining Rocket Pool to meet the demand for staking. When this occurs, Rocket Pool limits the amount of new ETH that can be pooled.

When Rocket Pool is at or near max capacity there is a chance your transaction will be rejected. To reduce the possibility that you try to stake and pay fees including gas for a failed transaction, the MetaMask Staking interface disables staking with Rocket Pool during these periods.