"Are you having fun in Denmark?"
Translation:Onko teillä hauskaa Tanskassa?
"Onko sinulla hauskaa Tanskassa?" would be correct, not sure if it's accepted but likely it is.
Perhaps you were thinking of a different case, where one would use the term "pitää hauskaa", which more or less means "to have a good time" in the sense of "partying". The following would be a valid sentence, but just not exactly what was asked for in this exercise:
- Oletko sinä pitämässä hauskaa Tanskassa? (translation: Are you out having a good time (meaning: partying) in Denmark?)
That's not even grammatical because you've used partitive for the subject predicative even though you've put the subject in nominative singular. More importantly, you've changed the meaning anyway, because adessive case is required to express possession of something.