Yeah, basically the RTP is a free download to offload the basic assets the engine uses when you make a game. Some creators will choose to not use most of this but the game won't work without those basic files. So the RTP is required.
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The company who makes the software provides the RTP downloads to mitigate this issue.
So in short you don't need to get the software, you just need to have the RTP for the respective game RPGM engine and system locale set to Japanese (Due to Unicode conflicts). The .net framework download page should've open up when it asked for it, install it to fix the patcher issue.
Again you just need the RTP to resolve it most of the time, I'm glad to help in anyway I can.
Stay safe and good night!
Till next time.