Why an analyser labels this move as a blunder? On the picture, you see the move that was offered by the analyzer.
My move was Na5. Forking the Queen and the bishop, so I can negate white's attack on f7 and defending the undefended pawn b7, so the queen can not take it.
However, the analyzer labels it as a blunder and offers Nd4. Why it's better? Why is my turn a blunder, where can I lose here material?
Your move 1...Na5 is a blunder because there was an opportunity to win material.

The suggestion 1...Nd4 is best because you win a rook, e.g. 1...Nd4 2.Qxb7 Nxc2+ 3.Kf1 Nxa1