In Catcher in the Rye, difficulties of communication are a constant issue. One way to understand this is simply seeing it as replicating a stereotype of the preparatory school environment, and particularly the experience of those from a slightly different class background than the majority of students in deciphering the unspoken codes of WASP society, in a way that parallels similar class mobility British works of the same period (Larkin, Osborne, Amis, etc.).
The lack of communication works in two ways as a theme in the novel. First, there is lack of attempts to communicate, that we see in Holden's reluctance to confide in his parents. The second is a deeper lack of communication, in which Holden talks but feels that people don't understand him (e.g. the episode with Mr. Spencer before Holden leaves the school) or people attempt to communicate with Holden, but he sees these attempts through a sufficiently hostile perspective that they never actually connect. Even Holden's flunking out represents a lack of communication between pupil and teacher.
Overall, lack of communication is a major theme in the novel. It's not really a symbol, as it doesn't stand for something else, and not a conflict per se, though possibly a source of conflicts.
Holden's lack of communication negatively affects his ability to form and maintain relationships with others throughout the novel. Holden's inability to communicate effectively also isolates him from the rest of society. Holden is seriously struggling during this period in his life and refuses to confide in those who deeply care about him. Holden refuses to speak to Jane Gallagher, who is the only person besides Phoebe that truly understands him, and continually seeks solace in shallow, selfish individuals. Throughout the story, Holden incessantly lies and misinterprets situations, which negatively affects his reputation. Other characters seem to view Holden with contempt because of his communication problems and are unable to form meaningful relationships with him. Holden's cynical nature and lack of self-awareness are also highlighted in his inability to communicate effectively. He is unable to look at situations from other people's points of view, which also narrows his understanding of the world around him. Holden's social skills deficit further isolates him from others, which adds to his mental anguish.