The exception is complaining about the slashes in the CSS-selector. The following is not considered a valid CSS-selector:
[value=DD/MM/YYYY]
To have slashes in the attribute value, you need to enclose the attribute value in quotes:
[value='DD/MM/YYYY']
Your method should work if you add the quotations inside the selector:
def select_date_format(format)
page.find("[value='#{format}']").click
end