Hi. I am currently trying to export a Rigged horse from the Apex Node to Unreal Engine for using it as a rigged character in the Unreal Engine. I tried different methods, but I always get same error. I am sharing the screenshot of the error and the setting I have kept for the export.

To be honest, I dont think the unpackchacter is fully compatible with the fbx character, I think you might need to unpack it further, also you are exporting the full apex rig which is the purple dot, and the base skeleton wichi is the grey one, try using the tiel as the base skin mesh, the grey as the base skeleton and the yellow as the animated skeleton, I don't know if you could export a rig from houdini to ue

The first output of the unpack character is a pass through of the character input. Output2 (capture mesh), Output3 (Skeleton) and Output4 (animation) are unpacked and is what you will use for the 3 inputs on the fbx character output.

One note, you have to add a prim name attrib to the mesh when exporting otherwise it will fail.

Can You use the apex rigging functionality in unreal? I thought it would only support the capturemesh, skeleton and animation

In tomorrow's Daily for Houdini 21.0 there will be a toggle on the APEX Unpack Character SOP that creates the name attribute on the output skin geometry.
Then you can wire those outputs directly into the FBX ROP.
Here's an example file.