When setting up a Spirit, what happens if the setup instructions are impossible (e.g., adding Presence to a land with Blight when that board has no Blight).

When setting up a Spirit, what happens if the setup instructions are impossible (e.g., adding Presence to a land with Blight when that board has no Blight).

Either don't start that Spirit on that board or follow the instructions as best as you can, choosing freely when no lands meet the criteria given. [Jagged Earth Rulebook p. 6]
When choosing freely, use theme as your guide — if you're in this situation, you're likely playing on the thematic map, after all!