Magento 2 Could not save product with position 0 to category
This issue has been occurring while you are removing the category from the Duplicate product. This is a known issue.
One of Our Client Faced this issue, and we have solved it by doing as following.
Step 1: Open Duplicate products in which you are getting this issue.
Step 2: Change the product "Visibility" to "Not Visible Individually".
Step 3: Then remove the category which you want to remove from this product.
Step 4: Save that product, and here You can see that the issue you were facing is solved now.
Step 5: Set product "Visibility" back to the original and save product again.