The default settings is not working here
▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀ Archivers ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▄
█
Active │ No █
Extension │ LHA █
OS │ Windows █▄
Description │ LHA (via 7-ZIP) ██
Pack Cmd │ 7z a -y "%1" "%2" ██
You have it disabled, so I would start by enabling it.
There are many different LHA archivers though so could still be problems. In this case you'll need to make sure you have 7Zip installed and if you can't type "7z" and see it work from your root Mystic BBS directory then you'll also need to add 7z.exe into your system path. (As an alternative you could also modify the Archive configuration to include the path to 7z too).
... Real Programmers balance their checkbooks in hex
--- Mystic BBS v1.12 A48 2022/04/26 (Windows/64)
* Origin: Sector 7 * Mystic WHQ (1:129/215)