i can't think of any reason to import a swf but if importing an mp4 causes a problem,
1. restart your computer and retry
2. if that fails,
reset your preferences - http://helpx.adobe.com/flash/kb/re-create-preferences-flash-professional.html
(or remove Win: C:\Users\<username>\AppData\Local\Adobe\Animate CC <version>\ and
C\Users\<username>\AppData\Roaming\Adobe\Animate\<version>\
Mac: /Users/<username>/Library/Application Support/Adobe/<Animate CC version>/ and
/Users/<username>/Library/Preferences/Adobe/Animate/<version>/)