When I convert the video to mp4 and play the video on a screen it gives the format is not supported
Find out what format (probably not just container – MP4 – properties but also codecs used to compress video and audio streams in this container) are expected by whatever device supplies picture data to the screen, then configure Avidemux (it should be the latest 2.8.2 nightly build or at least the latest 2.8.1 release) to output appropriate data.
Regarding MP4 specifics, do not select "Fragmentation" as streaming optimization for the sake of broader compatibility.
In general, it makes sense to supply information right away when asking for support.