Altough mp3 file is included in the list of supported audio files
i cant load any mp3 audio file when i do i get a File error
I can't be certain and I, myself, haven't had any mp3 import issues with Hitfilm, but I have with my Pinnacle studio and a few other programs I have used. I've found that there are a lot of variations of mp3s in the bits ranges and I have had to convert them to get them to read in those programs. What I have found is there are problems with the higher bit rates. 196 b/s etc. Usually 128 b/s works for me. Not saying that is your issue, but you might start there and see if it helps.
Convert it to WAV in audacity (free program). The problem may be the header or multiple tracks from what I understood in other threads..
The ID3 tags seems to be a common cause of these MP3 issues.