You get that error because WordPress filter some file types, including some audio file type such as m4a. To make it work you need to allow the file type you want to use for your episodes.
Option 1: Allow your podcast files in your code
wp-config.php file of your WordPress installation and above the line where it says
/* That's all, stop editing! Happy blogging. */ add the following code anywhere:
Option 2: Use a plugin to allow the files
There are several WordPress plugins that can fix the problem. Here are some plugins you can use to allow the file type you need: