There is an error in .discourse-compatibility
< 3.6.0.beta1-dev: 9f248e18b35e4cf599e35a871e9348957077e7c6
That commit does not exist so it does not install on 3.5.1 stable.
That’s what happens when you “fork” something but throw away all git history.
(EDIT I see that’s basically what @moin said)