Thanks for sharing the issue — you're not alone! The "Skill Level Maximum Error" and "Skill Level Probability" options were available in older versions of Stockfish like Stockfish 8, but they’ve been deprecated in modern builds (including stockfish.wasm, like the one used in lichess-org/stockfish.js).

The correct and supported option in recent versions is:

stockfish.postMessage("setoption name Skill Level value 1");

Recommendation:

If you're using lichess-org/stockfish.js, it compiles a modern stockfish.wasm, where only "Skill Level" and "Depth" are reliable for weakening play.

• "Skill Level": Ranges 0 (weakest) to 20 (strongest) — works reliably.

• "Depth": Very shallow depths like 1–2 can weaken it but can also result in unplayable behavior.