Make sure that the version in the OWSPlugin.plugin file is “EngineVersion”: “4.21.0” or higher. Then delete the Binaries, Intermediate, and Saved folders in the main project. Open the project SLN in Visual Studio 2017 (starting with 4.21.1 a new update of VS is required), set OpenWorldStarter as the startup project, then hit the play button. This usually works when I get that error. If not, let me know. Thanks!