In the "talks" section (on the right sidebar) there is an area called "settings". Here, you can choose who can send you messages. You can allow everyone to send messages, or restrict so that the sender must be at least newbie, at least pupil, at least specialist, etc. You can also block users.

The user who you are trying to contact probably has chosen a filter which excludes you. :(

you can send me message babe :) I will be there for you