The Merge Doesn’t Work

Yesssssssss(give it a like, I'm very late :( )

I make stairs

New Castle feature: https://castle.xyz/m/381501:7bafbcaf-1c3d-4895-8631-51b1697d168b?cxshid=HyHq8PW92

Finished

Why it does not merge ? am i doing it wrong ?