Where did you installed your Redmine application (what directory)? Please note that you need to indicate that directory as "installdir". Therefore, if you installed your stack at
/home/redmine for instance, you need to run:
./smtp-checker-v2.0.0-linux-amd64 -application redmine -installdir /home/redmine -mail_recipient YOUR_MAIL_ACCOUNT
Another issue could be that the tool is unable to retrieve the SMPT configuration from the file /opt/bitnami/apps/redmine/htdocs/config/configuration.yml. That could be for a bad structure on the yaml file. On that case, you can also run the tool indication the SMTP settings as shown below:
./smtp-checker-v2.0.0-linux-amd64 -smtp_host <SMTP HOST> -smtp_port <SMTP PORT> -smtp_user <SMTP USER> -smtp_password <SMTP PASSWORD> -mail_recipient YOUR_MAIL_ACCOUNT
You need to indicate the SMTP HOST, SMTP PORT, SMTP USER and SMTP PASSWORD that you configured on your Redmine stack.