"He goes to a world festival."
Translation:Ele vai a um festival mundial.
the portuguese grammar allows the use of "para" in the place only "a", they are fully interchangeable, you can use any of both words when one of them is used. The error here is the software is expecting a article, but the portuguese grammar allows the use of a demonstrative pronoun.
grammar rule: ir para (go for a long period). Vou para o Brasil (it means theperson is gonna move to Brazil). ir ao(go for a short period) Vou ao Brasil (for a short period, maybe for visiting). Although interchangeable in speaking, it is always good to follow grammar rules when writing! ;)
Not really sure what you mean in either comment...the sentence here is using the indecisive article in both EN and PT (a festival/um festival), so the decisive (o/a) isn't really in topic, I guess. Also, both the prepositions 'a' and 'para' would take an article, in this case the indecisive 'um'. (If festival was replaced by Italy or Brazil, the article would be the decisive as in 'a Itália' or 'o Brasil' because the country names are unique and decisive.) The choice of article, however, is not really governed by the preposition itself, and it would be the same for 'a' and 'para'.