The unmatched product warning will appear when you import your commission data that has a new products on it. The product will need to be assigned to a commission group to pay commission on items with the product.
Top menu >> Import data >> Unmatched product warnings
Under the warning, you will see the flag indicating new products are waiting to be assigned. Click on the Products warning
Click on the product you want to assign, and then select the commission group you want to assign it to.
If there is no matching commission groups, you are able to create new one by clicking the + Add a commission group button on the top left. For more information about commission groups, please see our article here.