Howdy,
The bracket means that (1) it was confused about what the chord name should be and (2) you're using an older version because that "feature" was removed in the latest version 2.6
If you're using a 2.x version of the software, you can visit the Redownload page at
www.notation.com/Redownload.php and get your download link for the most up to date version.
ttfn,
Sherry