Confused about Emrakul Promised End's Mindslaver trigger and upkeep stops

I cast [[Emrakul, the Promised End]] to control my opponent during their next turn. My opponent has a [[Sterling Grove]]. I set an upkeep stop on their side for their turn but I am not able to sacrifice Sterling Grove. Was I missing something? Maybe it works but the Grove just wasn't highlighted? It happened a while ago but I don't think I was able to activate it.