First of all, you can find every part of the sentence in a dictionary.
全部ダメダメでしょう？
That said, でしょう can be used in multiple ways. The fact that there is a question mark at the end of the sentence suggests to me that this is an instance of the confirmatory usage (see #2 in the linked answer). That's also likely why Google is giving you "I guess".
Additionally, you are simply not going to find some of words - such as I, they, etc. from the English translation in the Japanese text because the grammatical structure of the two languages are so different.
Realistically, でしょう by itself is likely accounting for both the verb be (are) and the I guess in Google's output translation. 全部ダメダメ has become they are all bad because I guess all bad is not grammatical English.
Lastly, I want to caution you about taking Google translate too literally. It is not a native speaker, and it is certainly not always right. It ultimately just makes statistically informed guesses based on a lot of other translations it has learned from. For particularly obscure uses of language which the neural network is unlikely to have seen before, you can often do better with common sense and a dictionary.
