It's worth mentioning that this isn't about opinions. Every question generates some opinion, that's just how interpersonal communication goes. However, we should focus on the main issue: primarily opinion-based.
A good place to look for the definition of primarily opinion-based is the Closed Question help page. It explains, like it does in the closed question, what it means:
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than on facts, references, or specific expertise.
What that means is that your question might have generated too wide of a variety of answers, mostly focusing on options or courses of action, rather than a more thorough understanding of a specific interpersonal problem or skill. For instance, a "What should I do?" question is primarily opinion-based because, rather than asking us for help with a specific case, you're asking for opinions on what decisions to make before we can even help with the deeper issue. We cannot make decisions for people, only advise them on how to carry on the decisions they've taken already.
Questions like "Should I have done X?", "I did X. Was I right?" or "What if X happened?" are also opinion-based in that they're not asking about interpersonal communication, but rather theories and/or confirmation. It is also a good idea to check out the Don't Ask help page for further insight into the topicality of questions.
A good interpersonal skills question doesn't have to be opinion-based. It can ask about approaches to a specific problem or type of problem, or how to improve on specific facets of social interaction, or what's generally acceptable under certain cultural norms. I highly recommend taking a look at this somewhat-related meta question. More specifically, the list of linked questions at the end of the question, which gives us some good examples of interpersonal skills questions that are not opinion-based.
