When I try to get a WebGL build, I get errors. It happens even in completely empty projects. I also tried to remove all packages (but Package Manager UI), but it still happens.
This is a known issue that happens when the OS is using a non-English locale (at least Turkish exposes it, and maybe others). We have a fix ready that we are hoping to release in 2018.3.5. Until then, please try switching the OS to use a English locale, and the problem should go away.