macOS has had (since 10.14) a built-in Split View mode that lets you use your full screen to display two apps side by side, each in a quasi "full screen" mode. Personally, I never use this feature—why limit yourself to just two windows?—but I know many people do.
In macOS Catalina, you activate Split View via a green button hover, which then shows this pop-up menu:
The activation method is somewhat different in 10.14, but the end result is the same—a window taking up half your display. Except when it doesn't…
As you can see, the full screen menu item works, but the two Split View entries do nothing at all—no error message, but nothing happens other than the menu vanishes.
Most users will never experience the above problem, as it only occurs if you've changed a seemingly-unrelated system setting: Unchecking the "Displays have separate Spaces" checkbox in the Mission Control System Preferences panel.
Apple notes this requirement at the very bottom of the Catalina (but not Mojave) Split View page. I've added a bit of emphasis to one section of the quote…
To use apps in Split View on other displays, make sure the “Displays have separate Spaces” checkbox is selected in Mission Control preferences.
There are two problems with this. The first is related to the bit of emphasized text: Apple's note makes it seem that you only need to have this setting enabled if you're trying to use Split View on other displays. But the above movie was recorded on my single-screen MacBook Pro. As soon as I toggled the setting back on, Split View worked perfectly. So this setting is required to be enabled to use Split View at all, regardless of the number of displays.
Secondly, the above note only appears on the Catalina Split View page, but it also applies to Mojave—again regardless of the number of attached displays.
This is a case where it seems like it'd be incredibly simple for Apple to provide a bit of feedback to the user–Apple is clearly aware of the need to have that setting enabled, and can easily check the setting. So why not display a simple dialog when a user selects one of the Split View options while the setting is disabled?
"To use Split View, please check the 'Displays have separate Spaces' box in the Mission Control pane of System Preferences." Apple could also provide a button to take a user directly to the right spot to change the setting.
Similarly, it'd also be nice if they warned users that disabling the setting will also disable Split View.
Annoyingly, changing the setting requires logging out and back in again, so it's a bit disruptive. But if you want to use Split View, and you've changed this setting in the past, you'll have to be disrupted.