Reels that use licensed music from Instagram's library can't be turned into an ad in Existing post mode. This is a copyright restriction from Meta itself, and no tool can work around it. Your options are to use Create new ad (which recreates the media as a dark post, but Meta may remove or block the licensed audio) or to publish a version of the video with original audio or no licensed music. If you want to keep the post's likes and comments, you'll need to repost the content with your own audio before boosting it.