Sorry for the inconvenience. We included the generate-certificate.sh script in our solutions recently and it seems those updated have not been propagated to the AWS Lightsail platform.
Could you please let us know the exact command you are executing in the command line?
For those solutions that don't have the generate-certificate.sh script, we have this section in our documentation that explains how to download the lego tool and generate the Let's Encrypt certificates. Please substitute DOMAIN and EMAIL when required.
Let us know if you have any questions