One thing you can try is to use VsFTPd, which should already be included in the Azure cloud image: https://docs.bitnami.com/aws/components/vsftpd/
If not, one fast solution would be to do the following:
sudo chown -R daemon:daemon /opt/bitnami/apps/drupal/htdocs
If you prefer to migrate your installation, you can try to make use of your same server, by disabling the Bitnami installation:
sudo /opt/bitnami/ctlscript.sh stop
Then, install the Bitnami Drupal Stack from here: https://bitnami.com/stack/drupal/installer
If it works, stop the newly installed installation, start the old one, and export the database as explained here: https://docs.bitnami.com/virtual-machine/components/mysql/#how-to-create-a-database-backup
After that, stop the old installation and start the new one, and import the database as explained in the "How to restore" database backup section.
Please let us know if you find any issues.