I have a error when configuring an imported avatar.

So I created an character and a rig in blender, I imported them then i configured them manually and when i hit the apply button this error appears:"Transform ‘Lower_Leg_L’ is not an ancestor of ‘Foot_L’ ".I searched on the internet a lot for this and I couldn’t find anything. If you know can you please help me fix this problem?

That error means the foot is not parented to the lower leg. Confirm the foot is in fact parented to the lower leg in blender. It might be parented accidentally to the upper leg or root bone.