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Everything you ask me here is a hosting question. I cannot answer these because I am not your host. In fact, you chose to be your own host. These are all questions you should be able to answer yourself. I cannot coach you into being your own host. That's way beyond the scope of the support I can offer, which is limited to my own software.

With that in mind, all I can do is tell you why everything you asked me is a hosting question.

[ Authentication failure ]

This means that PHP reports to our software that your (remote) server says that the combination of username and authentication method  (password, or key pair) you tried using were rejected. Usually this means one of the following:

• Wrong username
• Wrong password / key pair
• The user is not allowed to login remotely (or at all)
• Your remote server is not configured to allow connections from the network you are trying to log in from

It could also be something else. You are the host of the remote server, you should look at the error logs to find out.

Our server uses a remote/external MySQL DB server that is also located in AWS Lightsail

That sounds unrelated at this stage. Kickstart only extracts the backup archive which contains the restoration script which connects to the database. At this point in time you can't even run Kickstart. You are two steps behind the point where you'd need to worry about your database connection.

I cannot figure out where to put the kickstart.php file in lightsail server I have tried here: /opt/bitnami/apache2/htdocs

This is also something you should know as you're your own host, and that's the kind of information only the host can provide. I cannot possibly know that information.

I just did a DuckDuckGo search. It tells me the default document root is /home/bitnami/htdocs/. Or, at least, that was the case for a generic Lightsail instance in 2020, according to a random blog post that was talking about how to change that. Is it right? Beats me! I have no idea, and I can't be expected to have an idea about it as I am not using AWS Lightsail.

Which brings me to my next point. Don't self-host unless you have an in-house IT department or you know that you positively, absolutely, and categorically have the skills and time to configure, troubleshoot, and maintain all aspects of your hosting environment. If you are your own host it's up to you to solve hosting issues; nobody else can help you, unless you pay for a professional sysadmin by which point you might just as well host a hundred sites on a commercial host for a smaller total expenditure!