Ah, that's the case for projects written in C#, yeah currently Godot 4 doesn't support HTML export for C#. I wrote my project in GDScript so it was exportable.
Based on their blog post (https://godotengine.org/article/platform-state-in-csharp-for-godot-4-2/#web), it seems they're waiting for an update to the .NET framework that will hopefully make it easier to support web exporting C# projects in Godot 4.
Otherwise, the next best thing besides just waiting would be to make your projects using Godot 3, or to rewrite them in GDScript :/
Hopefully they'll do something about it soon.
