That's weird; what browser are you using? Does some error appear on the browser's console when clicking the button?
It seems there's indeed a limitation/bug on Electron when displaying the popup for creating a variable:
https://github.com/haroldo-ok/vcs-game-maker/issues/97
Okay, fixed the bug: the standalone Windows version is now creating variables correctly:
https://haroldo-ok.itch.io/vcs-game-maker/devlog/807901/version-0252-fixed-error...