'Invalid Recipient' message
This is a known issue; the error you're experiencing is only available in one place at the moment (as far as I'm aware - please correct me if I'm wrong) on the user profile. However, you can start a message from a couple of other places across the site.
Please click the 3 horizontal dots in the top right hand corner of any post or comment written by the user you want to message, and there should be the option to message the user. Alternatively visit your 'inbox' page, accessible via the drop down menu and click 'compose'. The name of the person you'd like to message should appear when you start to type their name in the 'recipients' box.