There are a few reasons why this could occur, but by far the most common is that one or more of the entries in the batch posted to a GL account that is set up as a consolidated account. You cannot unpost such a batch.

Make sure that all of your GL accounts are set to be detail accounts, not consolidated accounts, and you should be OK in the future.

Just for good measure, run Data Integrity Check on your GL data to make sure that the data is not corrupted.

Steve