If you install Unity Beta 2018.1.0b10 from Unity Hub, it won’t install the Unity.app to the correct folder, but rather to a /Unity/ subfolder that isn’t present in the normal install, not allowing Unity Hub to detect it.
Error when launching from Unity hub
Folder layout after installing 2018.1.0b10 from Unity Hub - Notice the extra /Unity/ subfolder
For reference, correct Unity 2017.3.1p3 path layout (not installed from Unity Hub, but recognized just fine)
Temporary Solution
Move “Unity.app” and “Unity Bug Report” up one folder (to /2018.1.0b10/)