At the beginning of each player's upkeep, that player returns a permanent they control to its owner's hand unless they pay 2 life.
A player who controls no permanents doesn’t need to pay 2 life (although that player can choose to do so).
A player with less than 2 life can’t pay 2 life and must return a permanent.
The choice of which permanent to return or whether to pay life is made as the triggered ability resolves.