As this error states, you are trying to add the playerAnim AnimComposer to the boots when playerAnim is still added to the player.
To get rid of this error, you would first need to remove the playerAnim from the player Node, but then your player will no longer be animated and things won’t work how you want.
The proper way to do clothing is to have a single AnimControl and SkinningControl for the player model. Then attach your boots to the player node that contains the existent animControl and skinningControl.
The last few replies I left in this thread should explain how to get animated clothing/armor working: How to add clothing to character model - #38 by yaRnMcDonuts