"I believe it is best if I do not enter."
Translation:Credo che sia meglio che io non entri.
'It is better' could be an alternative answer to DL...but "Best" cannot be "Il meglio" since meglio is an adverb and is not a noun. Migliore can be used as an adjective or noun to mean better or best È il migliore se..It is the best if.......È migliore se..It is better if ...È meglio se..It is best if OR it is better if.
Surely "best" here is precisely an adverb qualifying the verb "is", so that meglio is what is required. After all the "it" in "it is best" is an impersonal placeholder to get the verb going, so it makes little sense to qualify that with an adjective like migliore.
At least this way of thinking is not obviously wrong afaics (and is consistent with Duolingo's given translation).
The question from oktaya and MadelynWri's answer are part of what you're looking for. In Italian (so I was taught), the use of "che" in sentences indicating belief (“I believe”) or possibility of non-absolute events (such as ”I hope”) requires the use of congiuntivo (subjunctive). We've seen that once already (“sia meglio”); this particular sentence has two examples.