Translation:I believe it is best if I do not enter.
Keep in mind that Italian always uses the subjunctive after "credere" and "pensare," regardless of the amount of conviction behind the idea. So in English you can convey the idea following "I believe" or "I think" with the present tense if you have the present subjunctive; there's no need to add a "subjunctive feeling." The subjunctive in Italian does not necessarily need to be translated with a subjunctive verb in English (or with the conditional, which adds shades of meaning that are not there). At best you could perhaps go with "I believe it may be better...," but again, there is no need to do so.
In both English and Italian you can use different tenses in if-clauses depending on the degree of certainty. The present tense is perfectly fine here, and shows more certainty. For something more hypothetical (what's sometimes called a contrary-to-fact statement), you can use the conditional in the main clause (it would be). In Italian this would need to be paired with the imperfect subjunctive (sarebbe meglio se/che...entrassi...). This is a different structure with different shades of meaning, however, both in English and Italian, so I would recommend keeping the same tenses.
I hear you. It refused "I think it best that I not enter", insisting on "it's best", and it refused "I think it would be better" insisting on "may be better". Reported twice. Usually the English translations of Italian sentences are pretty good (unlike in some other DL languages that I won't name), but whoever coded this sentence doesn't seem to understand the subjunctive in English.
sergio: There is NO 'più bene'. That'd be like saying in English "more well". The comparative AND the superlative form of 'bene' is 'meglio'. Often the definite article in included when it means "the best", but it's context otherwise that indicates whether it's 'better' or 'best'.
Why not "I believe that it would be best that I not enter"?
With all due respect, this is a highly frustrating section because the course designers don't appear to have taken the time to learn how English indicates uncertain or potential actions with its own subjunctive (whether pure or in periphrastic form with auxilliary verbs).
The trick is to recognize whether the dependent clause (Che sia......) is in the indicative or subjunctive. Credo classically requires the subjunctive regardless of the level of conviction and it does so here. You can tell because the first verb in the dependent clause is sia and not è. The second verb is entri and looks like the 2nd person singular (indicative) and is the the same as the 1st AND 2nd AND 3rd person subjunctive. This does not address the other comments in this thread, and I agree that the English translation is not great especially shifting it to an “if” clause which in Italian it is not. Here is a conjugation of entrare.....scroll down to see the subjunctive (congiuntivo) conjugations. https://www.italian-verbs.com/italian-verbs/conjugation.php?parola=Entrare
Thank you so much for the prompt response. I'm so glad it's not me losing my grip on how to speak grammatical English. It seems to me that all subjunctive sentences in English are subjunctive in Italian, but not all subjunctive sentences in Italian are subjunctive in English - sort of all cats are mammals but not all mammals are cats. Anyway, I soldier on, grateful for your help.