I was running into this problem trying to get Achievements to work for LED. Calling Social.ReportProgress kept outputing cache count is 0 in the XCode and then whatever debug.log I told it to output in case of failure.
The answers came when I found this forum thread.
A few people on the thread even wrote their own plugins for Unity to get Game Center Achievements working.
Here’s one by Marcpeyre – GameCenterReportAchievementPlugin
And the other by TheSlayerNo1 – GKAchievementReporter.unitypackage
At first I couldn’t get either of them working …
I doubled checked the Achievement ID I was using and it turns out there are actually two different IDs in iTunesConnect.
One is assigned by the developer under your app, under GameCenter:
And the other seems to be some ID generated by apple under your apps main page.
At first I was using the ID generated by apple, and it was giving me the error mentioned. I then switched to the other ID and it worked. Success.
Also I was using TheSlayerNo1’s plugin.
Any questions, contact me.