This post proves 2 things:
1. You shouldn't be using an engine for anything but checking for blunders.
2. You don't have a basic understanding of chess principles.
6.Nd4 is a "mistake" in the sense that you moved the same piece twice. 6.Qd4 would have been the correct capture. It develops another piece, and prevents ...e5,
7.e4 is "inaccurate" because it allows 7...e5. Where 7.Bf4 prevents black from playing ...e5.
Basic understanding? This isn't basic understanding. If I lacked basic understanding, I would be playing 1. a4 and then ra3
I hadn't played chess in a year since May 2019 since a 0/5 performance. Starting May 2019, I actually tried improving at chess and in only 2 months, I went to 1242 uscf to 1558 chess.com rapid, which is equivalent to 1605 uscf according to here. https://chessgoals.com/rating-comparison/
Even if that conversion is wrong, it's a huge improvement. I just played a beautiful miniature today and I can see the improvement, chess.com said I played a mistake, 6. nxd4 and an inaccuracy 7. e4 on depth 20 but I think it's just engine's problem analysing openings.