Why is it not a checkmate? I am new to chess. I have tried to learn checkmate from the video lesson in chess24 website. There, in one question, it marked my answer wrong and said that it was not checkmate. Can anyone please explain the reason ?

I do apologize if I asked anything stupid. Thanks in advance.
Welcome to Chess Stack Exchange.
I believe you're doing well here.  It is surely a checkmate. Probably the website isn't programmed that way to recognize that move. But, as per your question, this is clearly a checkmate that resembles the one-rook-mate pattern.