There has been a glitch for several days such that most people, myself included, cannot reach out to someone and ask them to connect on LinkedIn.
It is requiring you to add a subject but there is no subject line to add it to! Below is a screenshot you see when you get the error.
Following my service ticket to the Linkedin Help Center (answered in only 2 hours, which is pretty darned good!), they told me it is known to their engineering area and will soon be corrected.
I will advise you when that happens.