The asset works perfectly in Unity 6 - I use it myself, and many clients are actively shipping Unity 6 games with it, including WebGL builds. In fact, the interactive demo linked on the store page is itself a WebGL build. This sounds like a configuration issue. Please reach out to seasidegamestudios@gmail.com with your build error details so we can get you sorted. In the meantime, make sure you haven't modified the Graphics project settings, and check out the Documentation.