It's not a bug. Sololearn added some criteria for the new learner to avoid messaging spam. So Dilshan Gamage keep learning, increase level, complete some courses and solve problem then you can unlock this feature.
I Am Groot !
That's a nice decision by sololearn and think is the reason here why OP is facing such problem.
But still I have encountered this problem recently with some people where I can't send message to them but they can. ( Only in very few cases ) and I am fairly certain that none of us is a new user.