We were discussing this on the other post. There are a couple of things to note:
We don't use a classifier for uncountable nouns.
Mây/cloud is special because it can be both countable and uncountable. "On cloud nine" = uncountable. A dark cloud = countable. So it can go either way, with or without a classifier. If you want to use a classifier, use đám mây = a clumps of cloud.
The use or non-use of classifiers in Vietnamese does not exactly correspond to the use of articles in English.
"I am cloud" in a sense similar to "I am wind" or "I am spirit" may be infrequent but it is not idiomatic.
This is nonsensical and not using the specific English article you wanted should not be marked as incorrect.
As you get deeper into the course it will become clear that DL's ideas on which articles to use in English do not always conform to English usage. As painful as it is to give DL what it wants, it is probably best to concentrate on learning Vietnamese rather than correcting DL's English.
Given that Duolingo relies heavily on translation, it seems impossible to separate "learning Vietnamese" from "having correct English." Just a thought!