This game is made with RPG Maker MV, which has a memory leak.
Best close your internet brower or other, RAM-intensive and not necessary programs when playing such games, as most GPU will be overclocked soon and then the RAM will be affected until the game or PC crashes.

Effect are high lags, graphical glitches, black screens.

If possible, disable some optional setting of the game to reduce the GPU usage.
If you have two GPUs, change the PC settings for the game.exe to the better one.

You can find the game.exe with right click on the library, show data files...

Have you checked if the game has been installed correctly?
Also see into the first pinned thread - it will answer your question.

How much RAM does this game need?

Nah, can't be that. I verified game integrity, and yes, my GPU supports WebGL.