You need at least 14,000 points for Brainly messages to work. The Brainly team has this enabled to encourages users to answer more questions which causes them to gain points. Brainly is also not a social site. They want the best answers, and for that, you get points. When you get 14,000 points, your messages will be allowed.
<u><em>DO NOT GO AROUND GIVING RANDOM ANSWERS. </em></u>I highly suggest you answer the best you can and to the fullest extent. If you do not, and you answer just for points, your answers will be deleted, and you will lost points.