When a sound can’t be used, its page has no Use this sound button. The reason is under the sound’s name instead.

What the page says

What it saysWhy
The author does not allow others to use this sound.The person who posted switched off Allow remixes for that video. Posts that already use the sound keep it.
This sound is no longer available.The video was deleted or taken down, it isn’t shared with Everyone, or the account is private. A moderator may also have taken the sound down.
This video has no sound to use.The video is silent or muted, or it was posted before sounds could be reused. Only a post with one video lends its sound. Photos and posts with several videos don’t.

In the sounds list when you add music, and in Saved → Sounds, these sounds say not available.

When your post fails because of the sound

A sound can be withdrawn after you picked it. Then the post can’t go up, and the card on Home gives the reason with Remove the sound, then post again. Tap Edit, take the sound off, and share again.

Your own sound

You can always use the sound from your own video, even with Allow remixes off, and even if the video isn’t public.