It seems that you don't have certificate files in the instance.
The guide that @gongomgra posted will help you to create them. If you followed that guide probably you don't have the files in the correct location. Maybe you forgot about the following commands (where DOMAIN is your domain):
sudo ln -sf /opt/bitnami/letsencrypt/certificates/DOMAIN.key /opt/bitnami/nginx/conf/server.key
sudo ln -sf /opt/bitnami/letsencrypt/certificates/DOMAIN.crt /opt/bitnami/nginx/conf/server.crt
Please try to to follow again the sections of the alternative aproach using the
Nginx section in the
Also, don't forget about the
Step 2, that step will generate the files you are missing.