In order to use your track in an advertised Instagram / Facebook post or reel, the track must be linked to your Instagram / Facebook profile.
You can find out how to link your track to your profile here.
Alternatively, you can also embed your song directly into the video beforehand—for example, using an editing app like CapCut - and then upload it that way without selecting the song from the library.
Unfortunately, Business Accounts cannot currently use the music stickers.