Where it says Script: ActionScript 3.0 in the Properties panel, click the button with the wrench icon to open the Advanced ActionScript 3.0 Settings window. Under the Source path tab, you may have added a folder that contains Feathers source files. If you only see "." or maybe "src", then that's not the issue.
Alternatively, your project may have a feathers folder in the root or maybe in a src folder.
Some example paths in your project might look like this:
Or maybe this:
Either way, you need to get rid of that feathers folder and all of its content. You're using the SWC, so you don't need the source files.