I don't see the issue with using directly rather than straight for "direto". In matter fact I believe it makes more sense in English utilizing directly, as it relates specifically to directions and time, rather than straight which could relate to various other things.
If someone could explain to me why it's directly is not accepted, I would appreciate.
"Straight" actually is an adverb (in addition to being an adjective and a noun), so duolingo's use of it here is entirely appropriate.
"straight . . . adverb 1 in or into a level, upright, etc position or posture • Is the picture hung straight? 2 following an undeviating course; directly • went straight home • looked her straight in the eye. 3 immediately • I'll come round straight after work. 4 honestly; frankly • told him straight that it was over. 5 seriously • played the part straight."
I wondered about the same thing, I think it's just the adverb "diretamente" devolving into the adjective "direto" because it's shorter. I've seen it with "rapidamente" -> "rapido". I wonder if it's considered correct in more formal contexts however.
I know we do something similar in french, although here it's considered informal. That is I could say informally "Nous allons direct à la plage" instead of the more correct "Nous allons directement à la plage". I can only hope it's the same thing in Portuguese.
For anyone struggling with the slight differences of "Vou à and Vou para" here is a link to help you out, I'm sorry the link is completely in Portuguese I still hope it helps! http://mundoeducacao.bol.uol.com.br/gramatica/vou-ou-vou-para.htm
"Nós vamos direto para a praia" can also have a different meaning in Portuguese. It can also mean
• 'Nós vamos sempre à praia' = We always go to the beach.
• 'Nós vamos à praia frequentemente.' = We go to the beach frequently.
Here, "direto" acts as an adverb that indicates doing something regularly.