The engine is recommending Qc1 and I cannot figure why it is the best I tried a lot of variants to induce checkmate or gain material by moving the recommended move Qc1. I tried it instead of Nc4, which the engine considers a blunder. However, I cannot solve why it is the best move.
[FEN &quot;&quot;]
[StartPly &quot;28&quot;]

1. e4 e5 2. Nf3 Nc6 3. Bc4 Qf6 4. d3 Nge7 5. Bg5 Qd6 6. Bh4 Ng6 7. Ng5 Qb4+ 8. Nd2 Nxh4 9. Bxf7+ Ke7 10. O-O d6 11. Bb3 Nd4 12. c3 Qb5 13. cxd4 Qxd3 14. dxe5 dxe5 15. Qc1 (15. Nc4)
Consider the position before 15. Nc4.
[FEN &quot;r1b2b1r/ppp1k1pp/8/4p1N1/4P2n/1B1q4/PP1N1PPP/R2Q1RK1 w - - 0 15&quot;]

You have developed two knights, a bishop, and castled. Your opponent has developed a queen and a knight, and his king is stuck in the center.
In other words, you have a massive dynamic advantage! You are already in position to leverage your superior development &amp; king safety and steamroll your opponent off the board. There are plenty of ways to do so - Rc1, Qc1, Qh5, they all produce immediate threats against your opponent's king that are hard to fend off. These moves even come with tempo because Black can hardly allow Qxc7+ or Qf7+.
Nc4, on the other hand, allows your opponent to trade queens. When you are on the attack this is one of the things you least want to do, because the queen is the most powerful piece and  hence also the most powerful attacker. After 15. Nc4 Qxd1, you still have a comfortable advantage, but you are no longer playing for immediate checkmate.
I haven't looked at the computer's suggestions and it probably sees something more concrete, but still: on general grounds, trading queens in this position is a bad idea. By extension, that makes Nc4 an inferior move.