Thank you for using Bitnami. You can click on "Open Terminal" to open a command line inside the VM. Then, you can create a directory there running these commands
mkdir -p /opt/bitnami/NAME
chmod 777 /opt/bitnami/NAME
This way, you will have a folder called NAME inside the /opt/bitnami installation directory and you will have write permissions inside that folder.
Let us know if that works for you