Hi Lotus,
Yes, Natalia is correct: it means that the person has difficulty growing close to certain people. In this case, this person has difficulty building solid relationships with adults and peers. Here are a few more examples:
"I could never be a teacher. I just cannot relate to children."
"He turned her down when she wanted to be his girlfriend, saying he could not relate to girls with her personality."
"The teen volunteering at the retirement home easily related to the elders, and now they love seeing him there every week."
I hope these help. Please let me know if you have any more questions.