Variations and add to cart button
It is not clear when you get to a product that has variations what to do, you assume its just intuitive they will choose a variation - but its not. Also the Add to cart button doesnt appear till after the variation is chosen. On oldnavy.com the add to cart button is always there. If I have not chosen a variation but I have clicked on 'add to cart' a little message appears 'please choose size/color'. For woocommerce, maybe that text could be whatever name you gave given for the variation group (ex. Color, mat color, size)
I have Time Zone variations for some of my products. The Add to Cart button is always there. A message pops up if they have not checked one of the options for the different version.
Look at this page:
The message isn't as specific as you mentioned, but it gets the job done.