If you open the [FJsonObjectConverter][1] documentation page and scroll to the bottom, you’ll see what Module it requires and which Header file to include, like so:
Then you need to open your YourGameName.Build.cs file and add the dependency module to the list. Something like this:
PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "JsonUtilities" });
Finally, you should include the Header in the files where you need the functionality. The directive should be placed near the other includes, at the top of the file:
#include "JsonObjectConverter.h"