I got this error when trying to install the latest Sierra update (10.12.5). I’d closed iTunes, and all other apps, before doing the update.
The workaround, for me anyhow, was to open iTunes, connect to the iTunes Store, then start the update. I got a popup saying iTunes needed to be shut down, and to click “Continue” if I wanted to do so. I clicked “Continue”, iTunes was shut down, the update proceeded normally, then iTunes was reopened. When iTunes opened, there was a message about the iTunes library being updated, so maybe the update had some iTunes fixes in it.
Anyhow, I’m passing this along in case anyone else runs into the problem. I found it via Google, so I’m not alone.