From your previous post, it looks like you’ve already installed DA for 4.21 from the launcher. Please do the following steps:
Make sure your previous version you manually added to your project is removed. Go to your projects folder and remove the GAME_PROJECT/Plugins/**DungeonArchitect **folder if it exists
Launch the Unreal Editor and navigate to Edit > Plugins and enable Dungeon Architect. Restart the editor when prompted to and try again