If you defined
direct in your wp-config.php file, you shouldn't get that message.
We would like to reproduce your issue in order to help you. Could you give us the exact name of the plugin that you are trying to install?
As I understand, you could successfully install and activate the plugin, but then, when you try to access wp-admin you get ftp credentials error. Am I right?
Also, could you run the following commands through a SSH terminal and paste here the output of them?:
ls -la /home/bitnami/apps/wordpress/htdocs/wp-content
ls -la /home/bitnami/apps/wordpress/htdocs/wp-content/plugins
We’ll be looking forward to receiving your reply.