Google added the ability to verify and merge your Google+ local pages with social pages a week or so ago. Many have already transitioned the pages over but many have yet to do it.

Those trying to do it today may not be able to because the feature is ‘erroring’ out with a 500 error response code.

Jade, the Google+ Local community manager said in a Google Places Help thread that they are aware of the issue. She wrote:

So sit tight, Google will hopefully fix it soon.

Forum discussion at Google Places Help.