This error occurs when you have associated a child SKU with multiple parent SKUs.
Invalid variation relationship: The child variation is already assigned to a parent SKU. Please delete the old relationship before submitting a new relationship.
If you upload an inventory file to your selling account in which variations have been created and want to reassign your child SKUs to different parent SKUs, you must first submit a delete feed to break the variation family that was created during the initial upload. You will need to do this before assigning a new parent SKU to a child SKU.