Why can't I move the knight on e2? Why does the engine suggest moving the knight on b1?
The reason you cannot move the knight is because it is blocking the check from the queen on e7. This is known as a "pin". Since it is illegal to make a move that puts yourself in check, the knight cannot move.
Even if that e7 queen was not there, moving the knight would also allow the bishop on f3 to take your queen on d1. This would also be called a "pin", because even if the knight could legally move, it would lose material if it did.
The other answers are suggesting playing the b1 knight to d2. I think this would be a mistake, since that would block your queen from defending d4, allowing Black to play Nxd4 (which would win a pawn, attack the knight on e2 one more time, and also protect Black's bishop on f3.) And if you actually take the f3 bishop with that knight, you end up losing your g1 rook to a fork, as I show in the diagram. Nc3, which develops a piece and defends the e2 knight, is probably better.
Rg3 is also an interesting idea. It attacks your opponent's bishop and sets up a threat of Re3, which would turn the tables and pin your opponent's queen to their king! Black can play Be4 to block that threat, but that also blocks the pin on your knight.
[FEN "r3kbnr/ppp1qppp/2n5/3p4/3P1P2/5b2/PPP1N2P/RNBQKBR1 w Qkq - 0 1"] 1.Nc3 (1.Nd2 Nxd4 2.Nxf3 Nxf3+ 3.Kf2 Nxg1 4.Kxg1) (1.Rg3 Be4 (1...Bh5?? 2.Re3!) )
Welcome to Chess Stack Exchange! You cannot move the knight because it's currently pinned to your King. In simple words, if you move your knight on e1 then your King is in danger because of the black Queen on e7.
You are suggested to move your knight from b1 to d2 because that would help in developing your pieces and would also attack your opponent's bishop.
Moving your e2 knight is illegal because it is in a position we call a "pin" It happens when your piece(in this case, your knight) is the only piece blocking the opponent's attacking piece(in this case, the queen) from checking your king. That prevents your knight from moving it, therefore pinning it.
And you are recommended to move your b1 knight because you attack the bishop this way. At the same time, this move helps develop your pieces. Hope I was helpful!