Product links in menu not working

I have 4 categories listed in the site’s main menu under ‘Books’. These categories are:

https://ukds.uk/book-category/learning-to-drive

https://ukds.uk/book-category/theory-test-practise-questions

https://ukds.uk/book-category/highway-code-theory-test

https://ukds.uk/book-category/glovebox-guides

For some reason the links to the category pages are not working. Each of these links just goes to the site home page?

The page I need help with: [log in to see the link]