What did God tell the people not to touch at Mount Sinai?

Answer: The mountain

God told Moses to set a boundary around the mountain; whoever touched the mountain would surely be put to death.

“And you are to set up a boundary for the people around the mountain and tell them, ‘Be careful not to go up on the mountain or touch its base. Whoever touches the mountain shall surely be put to death.”— Exodus 19:12 (BSB)