If you get an error message saying that the file is of the wrong type, the fix could be as simple as changing "MP3" (uppercase) to "mp3" (lowercase) and trying again.
If you get an error message that just says that the file won't upload, it is most likely because the file has corrupted metadata. Probably it's the playtime info that needs to be corrected.
In most cases, you can fix the file yourself. We suggest you use the free software MP3Repair or any other software where you can convert/repair/edit your mp3 file.
Upload your new file in Storykit. If you still have problems with your file, please contact support@storykit.io.