랑 is used like 와 and 하고.
얘기하다 is an abbreviation for 이야기하다.
랑 is used in speech specifically though and since this part of thr course is casual, that's probably why they used it. 와 and 하고 are used in writing. They should have clarified why the used 랑 here though.
서연과 이야기하자 > is not accepted but in another exercice 선생님과 이야기하자 is accepted... why is it wrong here? @duolingo: thanks in advance for the grammatical explanation