Cucumbers are subject to several leaf spot diseases. We can't determine which is affecting the plants by viewing the photo.  Sometimes laboratory analysis is needed to do that.

However, we think one of the three diseases discussed and illustrated in the following document is present.  By comparing the leaves on your plants with those shown in the publication, you might be able to determine which disease is present and how to manage it.

It may not be possible to slow or stop the progression of the disease this year, but steps can be taken to improve the health of the vines in the future.

https://plantpathology.ca.uky.edu/files/ppfs-vg-10.pdf