Actually the author of this post got confused by the development workflow of the strapi package, while he was trying to launch the strapi admin in development mode.
There is no need to run in development mode the strapi app and the admin panel. You should run only the Admin panel, as it already handles to run the app in the development mode.
So you should run just one command:
yarn develop --watch-admin
If you are still encountering some errors please try to delete build and .cache folders then try to rebuild it again and launch the Admin in development mode.