Get Attachments returns an Array, regardless of if there is 1 or 1000000
You should use an Apply to each to Iterate through the returned Attachments, and then use the Create File if you want to save it.
Also, in the future, you should be posting your flow so we can see what expressions etc you are using, as well as a copy of one that failed, with the error, but more importantly so we can see what you did, and then we can see the inputs and outputs of each action in that run.
But right now, unless you specifically are assuming only 1 attachment and specifically are writing the json path to get it, your flow needs to do what I said above.
Then you need to make sure you are saving the attachment bits.
Loads of examples of how to grab attachments and save them, probably 800,000 is my guess lol (like jelly beans in a jar), but seriously there are so many, I feel if you didn't see or find one, either a) your searches were bad (and thats not a dig at you its just a possibility) or b) you are missing something in what you intend to do and how you are explaining it is
Hopefully it just you needing to put it in a loop :-)
cheers!!!