To me, the sentence structure "(Subject) có thể... được không?" has a sense of politeness. To me, it means "Can (subject) please...?". However, I understand it can be frustrating for a word to seemingly appear out of nowhere. An alternative answer is "Can (subject)...?" but it kinda does lose that cordial touch. The good news is, we can all learn from this exercise =) I hope that helps!
I thought the same about the sentence structure, and therefore used the modal verb "could" instead of "can". It was marked incorrect, but should have been accepted. Although the word "could" is a past tense version of the word "can", it is also often used to replace the word "can" as a more polite way of making a request. Therefore, due to the structure of the Vietnamese sentence, and from what you have said about politeness, I actually think the word "could" is a better choice than the word "can", in this instance.