Require customer to choose attributes on product page not in cart. V2.4

Each of my products are available in a variety of colors and sizes.
The colors are NOT consistent between products.
My goal is to force the user to choose the desired size and color from the product page.

What I've tried:
     1.  Creating a Checkout Attribute:  this didn't work because
          a)  the colors are not consistent between all products' and
          b)  the user would need to remember the product color names from the product page  to accurately choose the color wanted from the cart
     2.  Marking "required" on the product variant attribute;  this appears to place an asterisk in front of the variant on the product page with nothing else happening.  In other words, the product can be added to the cart without choosing from the drop down list.

I'm thinking this must be something simple that I'm overlooking and will appreciate any help provided.

Thank you!

You don't need checkout attributes. You need product attributes. But marking it as "required" should work fine out of the box. For example, try adding this product on our demo site. You won't be able to do it until all required attributes are chosen

This was too simple.

What I did wrong, in case someone else has trouble:
I had chose Is pre-selected under a product attribute I created named ----- (I named it with dashes to indicate no selection had been made and chose Is pre-selected so it would always show up).  This was bad thinking on my part, it could be added to cart without any selection even though the variant was marked required.

The "dash" attribute has now been deleted and no attribute is chosen as 'Is-preselected'.
I also changed to radio buttons instead the drop down menu.  No button is pre-fills until one is chosen where I found the dropdown list pre-filled and the first item on the list was added to cart whether that's what I wanted or not.  I'm much happier with the radio buttons.

Now when a size or color is not chosen a red lettered attribute name appears in the upper right corner of the product page.  PERFECT!!!

BUT, I will be posting another question to find out how to get my attributes moved from the bottom of the Product Page to the box in the upper right corner where the red warning appears.

Thanks a lot for your help, it is greatly appreciated.