Before I say anything about the solution, please be sure follow the Troubleshooting instruction that written in official site of this plug-in
If this was not helpful for you and you still see the Error message, try the following sulotion
1. From your WP CP Go to "Audio Player" Options page.
2. Go to "Display" tab.
3. Click on "Reset Colour scheme" button.
but NOTE that your previous settings will be removed and NO WAY to retrieve it again after you click this button, if you have a customization, just write your values before reset color scheme.
This was everything about my solution of my case of my problem after 2 hours trying to solve this annoying problem.
Thank you and sorry for my English :')