I installed the plugin without any issues in a second bitnami instance account, so I guess there is something wrong with the first instance settings?
I tried manually creating wp-content/updraft with 755 permissions, but the plugin says:
Backup directory specified exists, but is not writable.
(Only if I set 777 permission is it writeable.)
I set up both instances within days of each other but I don't know what I did differently to cause this difference with permissions.
One bitnami instance allows the plugin to create and write 755 folders, the other bitnami instance doesn't.