In my menu section Sound & Video, Ardour5 is now added.

I opened Ardour. It guided me through the setup prior to first usage.

I completed that, accepting defaults.

I tried to start a New Session, but I got a dialog box saying

"Failed to start or connect to audio-engine.
Latency calibration requires a working audio interface."

I have not been able to get past that.

You can choose between ALSA sound system or the more advanced Jack sound server. I would recommend using ALSA for now. Select that as your "audio system" when creating a new Ardour session. Something like 44.1khz, 1024 buffer and 2 periods should be fine for most internal audio cards.

Later, you can play with Jack. Use qjackctl to manage jackd, it makes it easy to view your application sound routing and change settings. Also, if you have two jack compatible softwares you can use jack to start both applications. An example would be to use one application to play a composition or a video and another to record. Jack can start the recording and playing at the same time and handle all the routing as well. Very cool stuff can be done with Jack.

On a positive note, I am able to use Audacity, and I'm not in a big hurry.

Based on what you, Jason, wrote, I feel inclined to get Ardour working, so I can use this. While it's nice to have options, I agree that I may as well go straight to the best.

I'm looking forward to getting Ardour configured/set up, so I can use this for most everything I want to do with audio recording.

This might help you get started https://www.youtube.com/watch?v=arvPwZlU1ak

Here's what I'm doing now with video and audio editing. I will probably work on other projects in the future. This is just my current project. I want to offer my yoga instruction by video. So, I am recording videos of me doing specific yoga routines. When I have access, I'm filming from two different angles. I will discard the audio portion recorded while filming. Next, I do voice-over which is me speaking instructions about how to do what I'm demonstrating in the video. I might add a third audio track of background music. For this project, I think the audio editing is simple, and I can do this in kdenlive. I don't see any way to record audio in kdenlive. So, it may be best to record the voice-over audio in ardour.

Yes, ardour with xjadeo seems like a perfect solution for your use case. "Session" -> "Open Video.." This will import your video and add it to your audio timeline so you can do voice overs and back tracks in sync with your vid, etc.

Equipment I've started with now is my smartphone to record the video and the mic in my laptop to record my voice-over.

I think I found a Zoom H2 from Amazon.com for $170. Is there a problem with using the mic in my laptop?

Your phone and laptop mic are not going to sound very good. You might be able to fix this with some filters (normalize, eq, compression). I only recommend the H2 because it sounds great and is portable and simple to use.

You should look into condenser microphones for recording onto your laptop (ardour) directly. Maybe someone else has some experience with a inexpensive setup. I have a maudio fast track pro, it has phantom power for the mic. All this stuff can be found second hand for fairly cheap, just be careful for compatibility with gnulinux when buying audio interfaces. I know maudio is usually supported. I'm sure there is more supported hardware than when I was shopping for this stuff. The manufacturer won't be much help in this regard, so you might need to dig around the forums or try it yourself.

windscreens are your friend ;-)