Translation:Everything was ready, but I feel very stressed.
I agree. In explanations for other examples, it is stated that once the word 'đã' has established the past tense there is no need to repeat it. Indeed, in some examples of translation into Vietnamese repeating the word 'đã' is marked as wrong. This is another example of disappointing inconsistency in Duolingo, which does not help with our proper understanding of the language.
In context this sentence would not be a problem. Presumably, the person addressed would know the state of play and would have no difficulty applying the sentence to the situation.
The problem is in converting the sentence to English in which the tenses express some particular state of play that is left to context in Vietnamese.
I wonder whether "Everything has been prepared but I feel stressed " is a possibility.