Woocommerce – variable product issue (select option)

I have an intermittent problem when adding products to the cart. The products have 2 variables. One has a default, the other not. When the relevant options are selected and then you click Add to Cart, often there is a message displayed ‘Select option’.
Sometimes clicking the back button on the browser clears the error but it’s not a good experience.
I’ve also noticed the ‘clear’ button for the variables doesn’t work at the same time as the add to cart problem.
Any ideas what is going on?

https://ww.wp.xz.cn/plugins/woocommerce/