If you upload a pre-edited video with music already added to it, TikTok will flag your video for copyright. You will instead need to add music to your video using the in-app Sounds library.

It should also be noted that you currently cannot use more than 60 seconds of a track in a video.

If you cannot find your music within TikTok's Sounds library, this may be due to the account type you have selected. Please see here for more info: Why can't I find my music on TikTok?