He can write a message but I can't???

Someone wrote me a message so I want to answer but I can't. Every time I try it then I get this.
"Your message could not be delivered because you are not sharing a server with the recipient or direct messages are disabled on your server, the recipient only accepts direct messages from friends, or you have been blocked by the recipient."
He wouldn't block me because he wrote that he liked my posts.

Why can he write a message and I can't?