Periodically dgraph removes older versions of data(minTs is the minimum version we have available). This error means you are trying to read at a version which is not available. You would see this error if you are doing heavy mutations update and trying to read, in this case too many versions would be created for index keys and so dgraph automatically discards old versions and the transaction is trying to access older version of posting list.
But in your case either you had the transaction opened for long before reading while the mutations were going on or you delete the zw directory in zero which stores the latest timestamp(version) at which to read.