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?
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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.