Possibly cuke beetle damage, on older leaves this is expected but the damage isn't fatal unless they spread a wilt disease. . Inspect the plants for pests daily and kill these you find in the pics below...

https://www.google.com/search?q=cucumber+beetles&sxsrf=ALeKk00DASO8EDhYO2vkBKXpnrADlUAthw:1624829066070&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiqzrb637jxAhVXMlkFHUmVBfoQ_AUoAXoECAEQBA&biw=1280&bih=606

The upper right of that leaf looks like it might have some tiny caterpillar damage. have you examined the undersides of the leaves for caterpillars? I get tiny green ones that hide alongside the leaf veins and can be hard to spot. I smoosh 'em when I find 'em.

That said, it does look like an older leaf, and they can get a bit scruffy. If the new growth looks healthy, I wouldn't be terribly concerned.