You need to specify the GroupName in the .ucl file

Code:

GroupName=KF

Example of my Hello World Mutator:


Mutator=(ClassName=MutHelloWorld.HelloWorld,IconMaterialName=MutatorArt.nosym,GroupName=KF,FriendlyName=MutHelloWorld.HelloWorld.FriendlyName,Description=MutHelloWorld.HelloWorld.Description,FriendlyName=MutHelloWorld.HelloWorld.FriendlyName,FallbackName="Hello World Mutator",FallbackDesc="Log Hello World.")