If anyone still needs this…
I was building UE 4.22 from source in Visual Studio and got this problem.
This answer probably doesn’t pertain to the launcher version of UE.
PLEASE NOTE: I have no clue whether what I did was a good idea–just that it solved this problem at the time.
The solution was to build the UnrealLightmass project in Visual Studio.
To do that, I couldn’t just right click UnrealLightmass and build. (I got an invalid configuration for Win32 message.)
First, r-click UnrealLightmass > Properties. Click upper right button to go to the Configuration Manager. (Or just get to Configuration Manager any way you like).
In the list, find UnrealLightmass.
Change the configuration from ‘Invalid’ to ‘Development_Program’ (random guess) and Platform x64.
Then, r-click UnrealLightmass and builld.
Relaunch the engine for good measure.
This allowed the Swarm Agent to work in the Editor. But, again, I have no idea if my build configuration makes any sense.