I know you already got it working, but I think this info could still help you or someone else in the future
Since you solved it by creating a new IKRetargeter, it’s possible you had the Root retarget phase turned off by accident. You can toggle that on and off with these buttons in the top right of the IKRetargeter
Based on the warning you got, it’s possible you set the wrong retarget root (you should use pelvis on both rigs usually).
As for the animations not showing up, you need to make sure the source IK Rig and target IK Rig are in their proper slots in the IKRetargeter. If the target rig (the one with no animations) is placed in the source rig slot, there won’t be any animations showing up in the browser.