The 'Unity failed to build avatar' error usually occurs due to the following reasons:
Incorrect or incomplete rigging: If your character model is not rigged correctly, Unity may fail to generate an avatar for it. The avatar creation process in Unity depends on the proper rigging of bones and joints in 3D models.
Unsupported file format or corrupted model: Unity might fail to build an avatar if the model was saved in a format that Unity does not recognize or support, or if the model file is somehow corrupted.
Missing or misconfigured humanoid description: In Unity, avatars are typically created for humanoid characters. If the humanoid description is missing details, like which bone corresponds to which body part, or if the description configures the humanoid incorrectly, Unity might be unable to create an avatar.
Here are some potential solutions to the 'Unity failed to build avatar' error:
Review and correct your rigging: Make sure your model is rigged properly according to Unity's guidelines. All necessary bones and joints must be present and connected correctly.
Check your file format and model health: Verify that your model is saved in a file format that Unity supports (like .fbx or .obj). Also, check the integrity of your 3D model using a 3D editing tool to make sure it isn't corrupted.
Ensure humanoid description is set up correctly: When importing your model, ensure you've selected humanoid animation type and configure the avatar definition to match the actual structure of your model. Confirm that each bone has been assigned correctly in the Avatar configuration. If bones are missing or incorrectly assigned, you'll need to manually assign them.
Remember to always save your work and back it up before making these changes, as they can occasionally cause loss of work if something goes wrong.