If you're using a Business account on TikTok, you won't be able to add music or sound clips to your videos.
To enable this feature, switch to a personal account by navigating to the “Manage Account” section within the TikTok app.
For a full breakdown of the differences between Personal and Business accounts, check TikTok’s official guide: https://support.tiktok.com/en/using-tiktok/growing-your-audience/switching-to-a-creator-or-business-account
Additionally, if you upload your track directly as an original sound instead of using the TikTok Sound Library, your video may end up muted.
To avoid this, make sure to add your music through the official TikTok Sound Library.