Q:

FFMpeg Stream specifier no match

Im trying to combine 2 to n videos with various transitions with an audio from mp3 above them (last second is faded). 
ffmpeg -ss 5 -t 10 -i C:\Users\User\source\repos\tryRunFFmpeg\2_snow.mp4  
       -ss 5 -t 10 -i C:\Users\User\source\repos\tryRunFFmpeg\3_sun.mp4 
       -i C:\Users\User\source\repos\tryRunFFmpeg\outaudio.mp3
-filter_complex "[0:v]setpts =PTS-STARTPTS[va0];[1:V]fade=t=in:st=0:d=0.5:alpha=1,fade=t=out:st=9.5:d=0.5,setpts=PTS-STARTPTS+10/TB[va1];[2:a]fade=t=out:st=9.5:d=0.5[aa];[va0][va1]overlay[outv]"
 -map aa -c:a copy  -map [outv] -crf 10 -b:V 10M -shortest C:\Users\User\source\repos\tryRunFFmpeg\testAgaint.mp4

and I get this error
Stream specifier ':a' in filtergraph description [0:v]setpts =PTS-STARTPTS[va0];[1:V]fade=t=in:st=0:d=0.5:alpha=1,fade=t=out:st=9.5:d=0.5,setpts=PTS-STARTPTS+10/TB[va1];[2:a]fade=t=out:st=9.5:d=0.5[aa];[va0][va1]overlay[outv] matches no streams. 

A:

There are several minor problems with your command:

Stream specifier ':a' in filtergraph description ... matches no streams. 

Change [2:a]fade to [2:a]afade.  fade filter is for video.  afade filter is for audio. 

Filter afade has an unconnected output

Change -map aa to -map [aa]. 

Streamcopy requested for output stream 0:0, which is fed from a complex filtergraph.  Filtering and streamcopy cannot be used together. 

Remove -c:a copy.