That girl turned me down. How should I reply to this message?

Just ask her if she has any other plans or thinks it's not fun to be with you.

If she says she has other plans, just say I'll ask you out next time you are free. Although she may say that she has other plans when she is obviously free, it indirectly means that she doesn't like you, but you can assume that she really doesn't have time to play with you and then ask her out. )

If she says she doesn't like playing with you, it's too direct, so don't have any illusions. Unless you like her too much and don't want to give up, you should know more about her, so that two people can have more topics together, and maybe one day she will like you, too.