Thats odd, no one that playtested got this popup. These prerequisites are required to launch any Unreal Engine game, although I doubt you've never played a UE game before. It may be your prereqs are in an unfamiliar directory, or simply outdated. Regardless, you should just be able to hit "Yes" and it will install them. This should also help prevent this error from popping up with any other UE game in the future, including on older version of the engine.
I do want to thank you for bringing this up though. This was missed by me and my testing group, and it clearly can cause confusion. I will make note of it in the installation instructions. Unfortunetly its not possible to include them as part of the actual game, but I am glad to see that the installer prompt is working. That is unfortunetly all I am able to do.
As for a web package, not only are we unable to edit our submissions, but UE stopped supporting HTML since sersion 4.24. My game was built using 5.3, which means even if I had the ability to update my submission, a majority of the game would break and need to be rewritten for an older version of the engine. I did however learn just how many people use Web Based games, and I will try to keep that in mind going forward!