You must choose a target artifact or enchantment to activate Wickerbough Elder's last ability. You can't pay {G} just to remove a -1/-1 counter from it without a legal target.

If an effect puts Wickerbough Elder onto the battlefield with one or more +1/+1 counters, its -1/-1 counter and one of those +1/+1 counters are removed as a state-based action before you can activate its ability.