Very good application, but it seems that the video cannot be played after uploading

you mean in preview player or after encoding using ffmpeg video is unplayable? Can you give some more details about your browser, browser version, os, os version and pc/mobile, file, if its publically available that i can replicate. Any other info to debug would be helpful as well. Last but not the least i hope you have pressed play button and waited for some time like 2-3 seconds.