Why is a product sync showing when I try to manage products?

When you go to 'Manage Products' of any group, you may see a progress bar page showing a product sync.

A product sync needs to completed before you can change product group assignments.

If you are a running a batch import/export process where new products are being added, or modified, you will have to wait until that process is completed before you can do product group assignment / Manage Products.