"I'm buying six altogether."
Is the word order "一共我买六个" not valid? It was marked wrong. I thought in general, that it's okay to move one piece of sentence to the first position (as the topic). Is there some reason you can't do that here?
It's not incorrect, but that would seem like you are hesitating to think about how many you are going to buy, so it sounds to me like:
Why isn't "我一共在买六个" accepted? This say "buying" -- an action I'm in the process of doing. So shouldn't it be 在买?
Although you might be right in some contexts, I think perhaps you’re adding specificity that the original sentence doesn’t require. Chinese prefers simpler to more complex. So perhaps it’s a good exercise in reducing and focusing only on the essence of the sentence. Complete but concise. This is just a thought, not a definitive answer. You’d need a native speaker for that.
As a Chinese student myself, I have made that same mistake multiple times. You cannot use 在买. You use 在 when you are talking about a place or where something is or put.
In my mind there is no difference between 我一共买六个 and 我买六个一共. Both answers should be accepted.
Sorry to have to tell you this but I don’t think I’ve ever heard a Mandarin speaker use the latter.
As a Chinese student, there is a specific order to which a sentence in Chinese is put. Subject + Time + Verb + Object is one way to put it, I suggest you check out this website. https://www.fluentu.com/blog/chinese/2013/10/30/chinese-sentence-structure/