Failed to merge AVI files into one MKV file

Hello,
I receive the following error message using subtitle editor Aegisub.
The video file format is AVI and the problem starts when I try to merge them into one MKV file.
The Aegisub can no longer open the video.
Can you provide me any suggestion?
Thanks in advance.

Error setting video
Ffmpegsource factory: FFmpegSource video provider: Failed to index: Can’t create a parser Avisynth factory: AviSynth error: FFVideoSource: Can’t create the parser
