can't play Prime Video on 2012 MacBook Pro. Getting "Missing Plug-in"

Yosemite is the most resent version of OS X that I am allowed to load on this MacBook Pro (13-inch, Mid 2012)

Not so. Who told you that?

The Mid 2012 13-inch non-Retina can run as high as Mac OS 10.15 "Catalina." We have two; one runs Catalina and the other Mojave. No issues. See the 10.15 Catalina Systen reqs here:

https://support.apple.com/kb/SP803?viewlocale=en_US&locale=en_US

and the "About this Mac..." window from my Mid 2012 MBP happily running Mojave:

The problem with Prime is that the old versions of browsers with Yosemite may not support the current HTML5 video standards without a plug-in, but the 2012 MBP will if you simple update your OS.

You can get Mojave or Catalina here:

How to get old versions of macOS - Apple Support

If you have older versions of expensive software like MS Office or Adobe CS/CC, i strongly recommend not upgrading to Catalina 10.14 just yet. Old softwarwe won't run on Catalina. I suggest Mojave q0.14--it is friendly to older apps.

Hi there Deuce_Shotton,

We understand you're experiencing an issue with playing videos from Amazon Prime on your MacBook Pro, and we'd like to share a resource that can help.

Take a look at it here: If features on a webpage don’t work in Safari on Mac

Cheers!