I'm trying to change an MP3 file into an MP4 file and its saying that that it's incompatible? What might be wrong?
Please make sure that you have a compatible file type for the Output menu. If you add a file to the queue when the Output menu is set to an invalid output type (i.e. audio to photo), then even a valid input file type will show as incompatible.
If you are converting MP3 to a custom MP4 video, it will show incompatible until you adjust the dropdown menu in the Advance Output Settings panel on the right for Encoder from mpeg4 to libx264 (in the Video section).