Translation:I think that it is better for us to wait another thirty minutes.
If you mean the reason DL might reject the answer, it could be--but rejecting "I think it is better if we wait..." would be wrong. Native English-speakers often omit "that" in cases like this--in fact, including it here sounds more formal than most of us would be in everyday speech.
In the dictionary mais is not translated as another is: outro,um outro, outra pessoa, mais um, basically referring to people. Based on this I think that : thirty minutes more is correct. In some special situation another can be related to things like: "that's quite another thing" but it is actually meaning that two things are quite different. Como sempre as suas explicações são muito apreciadas.
I put "I think it's better for us to wait thirty minutes", and Duolingo says I need to add "more" or "another" somewhere (i.e. "thirty minutes more", "another thirty minutes"). I guess this is a more literal translation, but I don't think that extra word is necessary? It's not like it's ambiguous what the sentence means, and (in English, not sure about Portuguese) it'd be more usual to leave it out.
I think more / another is required because the Portuguese sentence has the word "mais" in it. If "mais" wasn't there, then I imagine "I think it's better for us to wait thirty minutes" would be accepted.
Then again, I put "I think it's better for us to wait thirty more minutes" and Duolingo didn't accept it since I wrote "for us to wait" instead of "if we wait", so maybe not. I used the feedback tool to tell the Duolingo staff that my answer should be accepted.
I just added a positive to offset someone's negative response to your valid question.
Anyway, I think that "more" or "another" is necessary. There's a difference between waiting thirty minutes and then waiting for thirty minutes more. Imagine that you have (already) been waiting thirty minutes and you decide to wait "thirty minutes more". In total, you will have waited for an hour.
Oh, I didn't mean to suggest it was wrong - it's just that Duolingo marks your translation into English wrong if you leave out the "more" or "another", and I didn't think that was necessary. Maybe I was irritated at losing yet another heart with an acceptably close (to me!) translation :p