I was having this issue so I looked into the source code. Auto retargeter works by applying ‘templates’ to the source/target mesh. If your character mesh does not match one of the pre-created templates, auto retargeter doesn’t know what to do. There’s quite a few templates available, you can view them in the engine file IKRigAutoCharacterizer.cpp. There’s one for Mixamo, Daz, CC4, UE4 manny, UE5 manny, and more. If you have a custom model or sourced a model from some uncommon source, there may not be a template available. And even if you do use a supported character, if the rig has any differences in bone naming hierarchy or indexes from the expected input it may not work properly (example: Mixamo template expects root bone to be named “Hips”, not “root”).
If you want to see if your character has a supported template or figure out why a supported rig may not be working properly, open IKRigAutoCharacterizer.cpp, find the template, look at the bone names, bone indices, and expected root bone name and ensure everything matches properly