Audacity doesn't recognize this tune's format..

How do I get Audacity to see this mp4 tune?..

I am running a fresh reset of Windows-7 in a Surface One… Audacity 2.3.2…

Do you mean the original “Microsoft Surface”? I thought that ran “Windows RT”.

Depending on where you live, it may or may not be legal to download / record from YouTube. You should check.

Audacity is not officially supported on Windows RT (including devices designed specifically to run Windows RT), so I don’t know if this will work, but instructions for recording audio that is playing on the computer, are here: Tutorial - Recording Computer Playback on Windows - Audacity Manual

I’m not concerned about legal stuff…
Are there any add-ons I can add to force Audacity to see this downloaded file?..

You have to install ffmpeg libraries into Audacity to rip the audio from mp4 video.

You can’t directly “download” from YouTube… There are ways to “capture” or “record” what’s playing, or 3rd-party websites that will capture and download for you…

So… Do you already have an MP4 on your hard drive, or you still trying to record it?

If you record it, you can save it (export it) in the format of your choice. Again You’ll need FFmpeg if you want to make export to MP4.