Why can't I send a friend request on Facebook?
When I can see his profile, and I can send a message, why is the (faded) button there for sending a friend request, but doesn't let me send one?
People can select the audience they want to receive friend requests from in their settings. If you know this person in real life, you might consider sending them a message and asking them to friend you.
Other thing is friend request button appearing/disappearing also depend on denied friend request or blocking then unblocking a user.