HTML5 missing function: pthread_create

You're trying to use threading in a WebGL app. System.Threading isn't supported because there is no threading support in JavaScript.

@Moana Actually, you may be using something that uses threading. I did encounter this problem; for me it was because of System.Xml.Serialization.XmlSerializer.

As you can see in the class file, it handles a given threshold over which the generation is done in background. The easiest way to fix this was to give the related environment variable a higher value than the default one ("50"), or no threshold at all :

 System.Environment.SetEnvironmentVariable("MONO_XMLSERIALIZER_THS", "no");

Of course, the issue can come from some other class entirely, that is also using System.Threading.ThreadPool.QueueUserWorkItem.