As far as I know, some research involves group-work.
If a PhD student is either introverted, bad at interpersonal communications or both.
What difficulties might they encounter?
Of course interpersonal skills are important in completing a PhD. As you note, a PhD some fields involves working in small to very large teams, where interpersonal skills are essential. Even in the few fields were individuals typically work alone, interpersonal skills are necessary to navigate relationships with peers, graduate advisors, other members of the community, etc.
This is not to say that with poor interpersonal skills one is doomed. Such skills can be learned, and one can improve substantially with effort. It may even be true that in some areas of science, interpersonal skills are less important than in other jobs. But to say that they are not needed would be a gross misrepresentation of what it is like to get a PhD or work in academic science at any level.