Some users choose to not to show their profile to other users, which is fine. When that is the case you will see the following image when you click their profile:
So there is no obvious button to send a message, but there is another way
1. Go the messages page
Click on your profile on the top-right corner and then on “messages”
2. Create a new message
And just type in the username of whom you wish to contact
and finally send!