intead of /trials. It's located next to /crates, or nearby.
The coordinates are 29, 145, -36. The Golden Knight NPC in that claustrophobic little entrance next to the mythical crate is the Ancient trials NPC. Just interact with him and your issues should be solved.
Hopefully this helps. If you have any further questions related to the ancient trials, please post it on this thread.
What Minecraft version are you on? Sometimes it'll do that because the version your on doesn't correspond with the version of blocks the ancient trials have.