We just launched an instance in the Bitnami GCE Launchpad, and we did not find any issue, in fact, the main user is already a Bureaucrat.
What version did you launch? Did you install MediaWiki by yourself? Just to know if there is any particular version of MediaWiki that was affected by any bug.
In any case, in order to fix this issue, you must execute the following in your MySQL console:
UPDATE user_groups SET ug_user='1' WHERE ug_group='bureaucrat';
Also, I recommend you restart your services in case that configuration is cached:
sudo /opt/bitnami/ctlscript.sh restart
If you don't have your MySQL root password, click here for resetting it, if not, you can use use the credentials from the file
I hope it helps!