Yasharn’s first ability can find just one basic Forest or one basic Plains card if you don’t have both in your library or if you don’t wish to find both.

If a spell or activated ability has a cost that requires a player to pay life (such as the activated ability granted by Lithoform Blight) or sacrifice a permanent (such as Kazuul’s Fury or Spare Supplies), that spell or ability can’t be cast or activated.

Other things may still cause players to pay life or sacrifice creatures, such as a resolving spell or ability.