Some users may get an error when adding a category due to their database user having insufficient privileges to lock tables.
If you can’t add a category turn on your site’s debug mode (under Joomla’s global configuration). Try adding a category again, if you get an error similar to this:
Access denied for user 'myuser'@'localhost' to database 'joomla' in /home/www/joomla/html/DOM2/includes/database.php on line 307
then your database user doesn’t have permission to lock tables. Simple Review needs to lock the Category table to keep the data structure consistent. If possible you should grant lock permissions to your database user via phpMyAdmin. If you are unable to do so then you can install this patch. As long as you don’t have multiple people adding categories at the same time you will be fine.
You can download the patch from here, unzip it and follow the instructions in README.txt.