When running the Alchemy v0.8.2.0 installer on Web 8.1.1 I get this error: "This access control list is not in canonical form and therefore cannot be modified".
This prevents Alchemy from installing correctly.
As mentioned in another question about DXA: DXA web-install.ps1 - access control list not in canonical form The issue comes from windows security settings on a folder.
All I had to do to solve it was to open the properties of the SDLWeb/web/Alchemy/ folder, go to the security tab. Then when trying to add permissions to one of the users listed, Windows will mention an error about the order or permissions and will ask you if you want to "re-order" them. Just click Yes and then give correct permissions to the various accounts: System, NETWORK SERVICE, CREATOR etc...
Run the installer again and it should run smoothly.