Hello,
This is expected behavior.
Swatches are designed to work only with variable products that have real WooCommerce variations. If a product has only one color and no variations, there is nothing for WooCommerce to switch between, so the swatches are not displayed.
Please convert the product to a Variable Product, add the color attribute under “Used for variations”, and create at least one variation. After that, the color swatches will appear automatically.
Best Regards