What did Joshua set up under the oak at Shechem as a witness to the covenant?

Answer: A large stone

Joshua set up a large stone under the oak near the LORD's sanctuary, saying it had heard all the LORD's words and would be a witness.

“Joshua recorded these things in the Book of the Law of God. Then he took a large stone and set it up there under the oak that was near the sanctuary of the LORD.”— Joshua 24:26 (BSB)