Ok, since this error is clearly not supposed to happen, may I ask you also tell me the version of .NET, and the contents of your .csproj file? I am not sure, but it may help to go into your project folder, go into the .godot folder and delete the "mono" folder inside. After that, try if recompiling fixes the issue. Please also check if there is only one assetplacer copy in your project, and that it is where it is supposed to be.