[I wanted to give crossfade tracks effects… so I separated the channels that were made in stereo. I split the stereo by pressing the Divide by mono. After selecting the whole area, I chose “Effect > Crossfade Tracks”. But the effect did not appear. just same!! Did I do it wrong? How do I get Crossfade tracks effect? 
https://manual.audacityteam.org/man/crossfade_tracks.html
I think you prepared your tracks wrong. I don’t think you need to split to mono. The effect works on stereo tracks.
Koz
That you went to the trouble of splitting the stereo-tracks suggests maybe you were trying for a panning effect, rather than a cross-fade … https ://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Amplify.2C_Mix_and_Pan_Effects
Oh~ I solved problem~ Thank you for your help 