"It is awesome that you went to Paris!"
Translation:C'est génial que tu sois allé à Paris !
First of all, Il est génial. Il is used when referring to a masculine subject, or as an impersonal pronoun. Here, it's neither case. It refers to that you went to Paris, so it has to be translated Ce (so it is = c'est). The use of Il est génial que is possible but unnatural and too literary.
Then, the second part and the subjunctive mood. I can't explain how to use the subjunctive mood in only a few words, but I will explain this particular case. It's called Valoration (Valuation). You are giving your opinion about something, so that something has to go with the subjunctive mood. You can also use the advice of relox84: 'all sentences of the type "Ce/il est [adjective] que..." are followed by the subjunctive', but that's not entirely true since when the adjective is not an opinion, you don't use the subjunctive mood. C'est vrai que vous êtes allé à Paris = It is true that you went to Paris; here the adjective is not an opinion, so no subjunctive.
There are many instances where the subjunctive is used even when there is no doubt expressed in the sentence. For instance, all sentences of the type "Ce/il est [adjective] que..." are followed by the subjunctive, which includes "C'est génial que vous soyez allé à Paris "
If you really don't want to use subjunctive, you can replace it by "de + infinitive" in any sentence of the aforementioned type, and the subject of the sentence will usually be understood through context:
"C'est génial d'être allé à Paris"
However duolingo probably doesn't accept this sentence, because "you" doesn't excplicitely appear in it, and due to the lack of context the subject may be interpreted as "we".