Incomplete comparison?

I have always taken exception with this aspect of the Grammarly service. In a very strict, very British sense, comparisons are supposed to be completed within the a single sentence (you find this in many British textbooks). So if you say "more", you are should complete it by saying "than what". This overly strict implementation of a "rule" ignores that comparisons are not always intended when you use more, less, upper, lower without saying "more than", "less than", and so on. It also ignores comparisons that are completed across several sentences or a paragraph.

I have found this aspect of the Grammarly service to be almost always wrong.

I’m not sure what the incomplete comparison means.  Perhaps in context with the surrounding sentences, it would make more sense to me.  I do see a couple issues with your sentence, though.

Bring probably isn’t the right verb choice, as it shows movement from somewhere else to here.  The event brings the foreigners.  But the opportunity isn’t somewhere else first & then brought here.  I would use creates or provides.

“Their” is the wrong pronoun for this sentence.  The foreigners already understand their cultures.  You want them to understand the culture of the host country.  A country is an it, not a they.

Since you are talking about the host country (singular), culture should not be plural.

The second advantage for the host country is that it creates a great opportunity for foreigners to understand more about its culture.