Why this function won't be fired when trying to use external API?

Why this function won't be fired when trying to use external API?

According to This, I can manipulate many functions using youtube API.
Now, this seek is working perfectly!
<button name="test" onclick="ytplayer.seekTo(30,true)">Start</button>
But these setPlaybackRate won't work somehow.
Why?
<button onclick="ytplayer.setPlaybackRate(+=0.1)">playbackRate+=0.1</button>
<button onclick="ytplayer.setPlaybackRate(-=0.1)">playbackRate-=0.1</button>
I tried these, too. But it didn't work either.
<button onclick="ytplayer.setPlaybackRate(2)">playbackRate+=0.1</button>
<button onclick="ytplayer.setPlaybackRate(0.5)">playbackRate-=0.1</button>