How to unlock burst mode?

I’ve never been able to get a background ost because I can’t figure out what this is. Every time I check no matter what ost it is it always says it’s unavailable, so how do I make them available?

I’ve never been able to get a background ost because I can’t figure out what this is. Every time I check no matter what ost it is it always says it’s unavailable, so how do I make them available?