I am a foreigner learning to play chess with my computer. The blue move is the last move and the red one is suggested to go.
I wonder why I can't move my knight anymore. The computer doesn't allow me to.
Chess Stack Exchange is a question and answer site for serious players and enthusiasts of chess. It only takes a minute to sign up.Sign up to join this community
Indeed, if this is a normal game of chess, then here White can play Nd5-c7. In fact that looks like the best available move. The red move Rxa5 actually looks like the worst available move. One issue is that in this position, every move by White is winning. White is already a queen up, which is a huge advantage. Even Rxa5 is not bad enough to throw away White's winning advantage.
However, I suspect that you have inadvertently selected a chess variant such as "suicide chess". In suicide chess, the aim is to lose all your pieces, and it's obligatory to make a capture whenever you can. In this position, the only capture available to White is Rxa5, so White would be obliged to play that move. This would also explain why Black has apparently played such terrible moves in the game so far.
By the way, the convention is that the file (denoted by a letter) precedes the rank (denoted by a number) when naming the squares on the board. So d5 and c7, rather than 5d and 7c.